The Hong Kong economy and society developed rapidly in the 1970s. The Government actively implemented initiatives in improving people’s livelihood, in particular, more focus was put on children welfare. Children were benefited from 9-year free education to learn and explore knowledge in different disciplines and aspects. Besides, they enjoyed a wider variety of childhood games and funs for their leisure time, such as playgrounds, toys, comic books, etc. Children’s TV shows and songs show children’s life in those days and allow us to recapture precious collective memories in our pastimes.
Cheng Kok-kong has written many songs for children over the years. As a primary school teacher, he loved writing children’s songs, with educational elements imprinted in the lyrics. He first participated in a recreation and leisure TV show in 1971 for handicraft skills demonstration. Then, he got the chance to engage in the production of a children’s TV show. He suggested the director to feature children’s songs in the show, and recommended himself for writing and rehearsing for those songs. Children’s TV shows provided him opportunities in lyrics writing and performance of children’s songs, and enabled him to develop further opportunities in lyrics writing such as songs for Cantonese dramas in variety show “Enjoy Yourself Tonight”.
Cheng Kok-kong began to take an interest in writing children’s songs for children’s TV shows in the 1970s. At first, TV broadcasters produced children’s shows merely for fulfilling the requirement of the service license. With the success of “Hopscotch” aired in 1976, more children’s TV shows were produced over the years afterwards, such as “430 Shuttle”, “Lightning Fax Machine”, etc. Owing to the hit “Hopscotch”, the show host Sunny Wong and the children cast, as well as children’s songs of the show enjoyed immense popularity among children. A popular trend of Children’s songs was thus developed.
A promotion campaign for “Hopscotch” was launched by the TV broadcaster during 1977 to 1979. A series of three audiotapes with lyrics of the children’s songs of “Hopscotch” were produced in 1977, 1978 and 1979 respectively and delivered to the public for free at Lee Theatre. The songs were sung by Sunny Wong and the children cast of “Hopscotch”, with music by Sunny Wong, lyrics by Cheng Kok-kong, and musical ensemble conducted by Joseph Koo. The themes of the songs were related to kindergarten learning, including :
- (i) habits in daily life;
- (ii) festivals and seasons;
- (iii) games and stories;and
- (iv) morals and ethics.
The children’s songs of Cheng Kok-kong are simple and easy to understand. He thought that the lyrics for children’s songs should be written from children’s perspectives, and should fit the target listeners, no matter children younger or older. Many of the lyrical works of Cheng Kok-kong are still applicable for today’s children education and reflecting the social context nowadays., 
Cheng Kok-kong has written Cantonese lyrics to a number of TV cartoon theme songs, including “Dr. Slump” (1983), “Caption Tsubasa” (1984, 2001), “The Smurfs” (198?) and “New Ninja Hattori” (1983), etc. He penned new lyrics in Cantonese for the original theme songs from these foreign cartoons, mostly anime from Japan. Without copyright restriction in early years, he also composed Cantonese lyrics to many foreign folk songs.
Apart from children’s songs, Cheng Kok-kong has also devoted himself in writing scripts for children Cantonese Operas. He began his pursuit as a professional lyricist for Cantopop and children’s songs for years before he got the chance to write scripts for Cantonese Opera in the 2000s, though he had been passionate about Cantonese Opera since he was young . He thought that typical Cantonese Opera such as “Double Suicide” and “The Marriage of Tang Bohu” may not be suitable for children to enjoy and learn. To foster appreciation for the art among children, he wrote Cantonese Operas for children that were suitable for their age, knowledge and language ability. As training on basic skills and techniques of Cantonese Opera would take at least several years and the number of children learners on Cantonese Opera was still low, the performance of children Cantonese Opera was quite difficult to put on production. Some of the children Cantonese Opera composed by Cheng Kok-kong are not performed on stage so far.
-  根據香港工商日報的一則小讀者投搞，無線電視的《星期六樂園》啟發兒童智慧；與麗的電視同類型的《兒童世界》相比之下，前者「好」很多(「星期六樂園是益智節目」，香港工商日報第9頁，1974年3月1日)。
- 〈讓我休息一天〉(1979) 由路家敏主唱、松任谷正隆作曲、鄭國江填詞。
- 〈七歲了〉(1979) 由路家敏主唱、鍾肇峯作曲、鄭國江填詞。
-  有關鄭國江與粵劇的淵源，可閱讀文章「我是從大戲中走過來的」《詞畫人生》，鄭國江著，2013年，頁169-172)。
In the 1980s, Cantopop was immensely popular. While romantic love songs were dominating the market, there were also songs of other themes. At that time, the Hong Kong economy was rapidly developing. The general public were optimistic and confident about the future. Government departments and social organizations launched themes songs for propagations and campaigns, encouraging people to strive for a brighter future, stay positive and determined.
Cheng Kok-kong wrote songs with a variety of inspirational themes, mostly for Government departments and social organizations. From being a professional lyrist to recent years, he has been working closely with different Government departments and social organizations to write lyrics for theme songs, including the anti-drug series of Treasure Beautiful Life, For Tomorrow, The Perfect Future and All for Love, A Fruitful Life for Hong Kong Independent Commission Against Corruption, and March On for “Hong Kong Diamond Jubilee Jamboree” to commemorate the 75th Anniversary of Scouting in Hong Kong. The lyrics of these inspirational songs contain beams of positivity that are able to lift the spirits of listeners, encouraging them to strive for a brighter future.
Cheng Kok-kong first wrote about opium poppy (botanically classified as Papaver somniferum) in his anti-drug songs. Opium poppies are the key ingredients to produce opium. As early in 1945, the sale or smoking of opium was illegal. In the late 1970s, the use of opium was greatly declined. Cheng Kok-kong had used opium poppy twice in the titles of his anti-drug songs, alerting people of the dangerous and poisonous nature of opium poppy behind its beautiful outlook. The two songs were sung by Cheng Kam Cheong in 1977 and Chelsia Chan in 1979 respectively. Except for mentioning cannabis in his anti-drug songs related to opium poppy, Cheng Kok-kong seldom included drug names, or explicitly persuaded drug users to quit drugs in the songs. He thought that this kind of songs should be written in a mild and subtle approach to encourage listeners to live a positive life and stay away from drugs, while direct anti-drug slogans or phrases should be avoided. For example, in All for Love, the chorus was written like a love song, and the verse encourages listeners to move on and start a new charter in life or implicitly persuade people to stay away from drugs.
Cheng Kok-kong was also invited to write songs related to social events such as the pre-handover emigration boom, fight against the SARS epidemic and disaster relief campaigns, etc. He wrote Love for the campaign in support of the victims of the Indian Ocean tsunami in 2004. In 1997, he wrote the theme song Convergence of Lights for the “Dreams of Hong Kong” campaign launched by Radio Television Hong Kong. There are two versions of Convergence of Lights, both with music by Chris Babida and lyrics by Cheng Kok-kong. Both of the versions encourage Hong Kong people to stay and build a better Hong Kong together. One version was sung by the choir of Radio Television Hong Kong, and the other version was sung by various local singers including Sam Hui, George Lam, Anita Mui, Alan Tam, Roman Tam, Paula Tsui and Sally Yeh.
In 2013, the community initiated the “Operation UNITE” to build up momentum and rally community efforts to fight together against the SARS epidemic. Cheng Kok-kong wrote the theme song Hong Kong Heart for the campaign, which was sung by dozens of singers from the eight major recording companies, and was broadcasted frequently by the six major local media. In writing the song, he used the lines “We shall overcome, we shall overcome; we shall overcome someday” from We Shall Overcome. The royalties for public performance of the music were collected by the We Shall Overcome Fund, USA. Therefore, he had written the lyrics on a voluntary basis in reality. When writing songs related to social events, Cheng Kok-kong would make encouragement and give people hope in his lines, but he would seldom explicitly write about the event.
Apart from theme songs for campaigns and songs related to social events, Cheng Kok-kong also conveyed inspirational messages in children’s songs and theme songs for TV dramas, such as the theme song Oshin for TV dramas "The story of Oshin” and Triumph in the Skies for “Triumph in the Skies II”.
-  鄭國江訪問，2019年6月19日。
- 〈一切為愛〉(1985) 由顧嘉煇作曲、張德蘭主唱。
- 〈錄香港心連心唱片 共四首歌五個版本〉，《華僑日報》，1990年5月8日。
〈香港心〉由鄧建明作曲、鄭國江填詞，而本地六大電子傳媒包括亞洲電視、有線電視、商業電台、新城電台、香港電台和無綫電視。有關活動資料及參與演唱歌手名單，可見〈六大傳媒齊響應 全城抗炎心連心 - 新聞稿〉，載《鄭國江詞作手稿特藏》。
 自1960年代，〈We Shall Overcome〉的版權費皆納入Highlander Research and Education Center的We Shall Overcome Fund；直至2018年，〈We Shall Overcome〉被納入公共領域(public domain) ，讓任何人都可以自由使用。有關歌曲被納入公共領域，可見〈‘We Shall Overcome’ Is Put in Public Domain in a Copyright Settlement〉，載《The New York Times》，26.8.2018。
Music for movies and TV dramas includes songs and background music. For songs, there can be main theme songs, interlude and tail end theme songs for movies and TV dramas. Although most of these songs are related to the stories of the movies or TV dramas, these songs can become hit songs alone, and contribute to the development of Cantopop.
Lyrists usually write theme songs based on the stories of the movies or TV dramas, using their own imagination, and from their perspectives. As the duration of most movies is about 2 hours, lyrists can just watch the movies and write the lyrics. For TV dramas, there can be dozens of episodes. Lyrists usually write the lyrics at the same time when the daily shoot is being carried out, instead of waiting until the completion of the shooting. Cheng Kok-kong started to write Triumph in the Skies once he received the description of the story, characters and outline of TV drama “Triumph in the Skies II” (2003).
No matter theme songs for radio dramas, movies or TV dramas, the final version of lyrics is usually decided by relevant parties such as the director and singer. In general, the lyrist will write different versions of lyric content based on the story. The director will decide the version to be used, and the lyrics may be modified according to the singer’s request. For example, Cheng Kok-kong wrote eight versions of lyrics for Light of Million Hopes, theme song for ATV drama “Light of Million Hopes” (2003), and ATV chose the first version finally.
Cheng Kok-kong also wrote theme songs for horror movies, including “Life After Life” (1981), “Mr. Vampire” (1985) and “Abracadabra” (1986) etc. In writing Ghost Bride, the interlude for “Mr. Vampire”, he included delightful lines for the horror theme, “Her eyes, her eyes, shine like stars”. Ghost Bride was not sung by traditional singer, but performed by the Jie'er Choir, a children’s choir. The children’s voice created an even more spooky atmosphere for the movie, contributing to the success of the movie and the popular trend of “jiangshi” movies in Hong Kong during the 1980s.
Cheng Kok-kong has written songs for different religions, including those of Catholicism, Christianity, Taoism and Buddhism. Not adhering to any religion and knowing people with different religions, he keeps an open mind to different religions. To him, religion is all about “Heaven”. As he puts it, “In Heaven, there exists all rulers of the universe whom I am to believe in.”.
Cheng Kok-kong has taught in a Catholic school for more than three decades. He goes to church every week with his wife, who is a Catholic. With these experiences, he was versed in writing lyrics for Catholic songs, such as lyrics for the recent “Matteo Ricci The Musical”.
His first encounter with Christianity occurred when his teacher and the teacher’s father who were both evangelists, shared the Christian faith with him at his young age. These experiences enabled him to learn more about Christianity. In 2011, he hosted lyric-writing classes on Christian hymns, teaching the skills in writing rhyming lyrics that fit into the music and penning new lyrics to the Christian hymns.
Cheng Kok-kong has written songs about Buddhism and Taoism. Apart from the traditions of chanting of prayers and reading of scriptures, Buddhist and Taoist organisations will propagate their religious beliefs and cultures through special events and campaigns. He was invited by some Buddhist and Taoist organisations for the commissions to write theme songs for such occasions., , , , 
Although religious scriptures are usually not easy to understand, Cheng Kok-kong wrote simple lyrics about people’s daily life and used his personal experience in writing the religious songs. His mother and great-aunt made Buddhist prayer and worshipped the ancestors. His great-aunt even woke up at 3:00am every day to read the scriptures. In composing From Our Heart, Cheng Kok-cheng got inspiration from the verses at the shrine for ancestor worship at home when he was young. He conveyed the message “reap what you sow” in the lyrics, and encouraged people to cultivate their spirits in a mindful way.
“We keep learning in our everyday life though we may not realize. Nevertheless, have you accumulated your knowledge? Have you ever made use of the knowledge?” said Cheng Kok-kong. He wrote the religious songs with a simple and down-to-earth approach, with his philosophy to learn in everyday life and apply the knowledge in life.
-  《鄭國江：香江填詞人》數碼人文計劃的的訪談實錄，2019年6月19至20。
-  音樂劇《利瑪竇》由黎允文作曲、鄭國江填詞、劉松仁創作及執導，2019年4月20至28日於香港文化中心大劇院首演。
- 〈大德昭四方〉(1993) 是天壇大佛開光大典主題曲，由于粦作曲、鄭國江填詞、譚詠麟主唱。
- 〈嗇色園關心你〉(199?) 由黎小田作曲、鄭國江填詞。
- 〈食德慈愛〉(2015) 是健康素食日主題曲，由盧冠廷先生作曲，歌曲原名是〈食素好〉。
- 〈一畝心田〉(2018) 是「一畝心田梵音慈善音樂會」主題曲，由陳雋騫作曲、鄭國江填詞。
- 〈愉快笑一笑〉(2019) 由黎小田作曲、鄭國江填詞、林利主唱。