Qianjin Yaofang by Sun Simiao of the Tang Dynasty is a comprehensive collection of medical classics. The original version has been lost. Zhenben Qianjinfang, as the earliest extant version, contains a large number of common characters and has a very high reference value. This article takes the common characters in Zhenben Qianjinfang as the object of investigation, and uses research methods such as document collation, Chinese character studies, and exegesis to comprehensively sort out and systematically describe the common characters in the book, in order to provide reference for the study of ancient medical books. Based on preliminary statistics, there are a total of 661 pairs of characters that have a relationship between orthodox and secular in Zhenben Qianjinfang. Referring to Mr. Zhang Yongquan’s classification method of vulgar characters, these vulgar characters can be divided into nine categories, including changing meanings, changing sound symbols, simplification of glyphs, multiplication of glyphs, writing variation, splitting or merging of components, displacement of components, newly created vulgar characters, and borrowing pronunciation and common words, etc. For each type of common characters, typical examples are cited for critical analysis to explain the formation mechanism and rationale behind the glyphs of the common characters. To sum up, the formation of the common characters in Zhenben Qianjinfang is mainly the result of the combined effects of two internal and external factors. The evolution of glyphs, the need for semantic expression, and the phonetic characteristics of Chinese characters have contributed to the emergence of vulgar characters within the language system, while external factors such as politics, technology, and culture have also had a certain impact on the formation of common characters.