Zhengzhang was born 郑祥芳 (pinyin: Zhèng Xiángfāng) in Yongjia County, on the outskirts of Wenzhou. As 祥 and 尚 have the same pronunciation in the Wenzhou dialect, his personal name became 尚芳 (Shàngfāng). While he was in high school, his parents changed the family name to 郑张 (Zhèngzhāng), a combination of their surnames.
At this time, he became interested in historical phonology, and studied the works of Yuen Ren Chao, Wang Li and others in Wenzhou library. In 1954, unable to enter university to study linguistics, he began geological work in the Beijing area. In his spare time, he continued to develop his own ideas on Old Chinese phonology, particularly the finals and vowel system. In the 1960s and 1970s, he undertook dialect survey work in Wenzhou for Lü Shuxiang until he was sent to work in a factory during the Cultural Revolution. During a period when the factory was closed due to a factional battle, he began exchanging ideas with Pan Wuyun and Jin Shengrong, and refined his Old Chinese system to a six-vowel system. Essentially the same system was independently developed by William Baxter (building on a proposal by Nicholas Bodman) and by Sergei Starostin. In 1980, he joined the Chinese Academy of Social Sciences.