Based on the rich experience in union cataloging of traditional Chinese medicine （TCM） books over the past 60 years， this paper condenses and puts forward the concept of “union cataloging”， and introduces the four steps which are establishing an organization， drawing up a plan， collecting data， and summarizing and proofreading， as well as three aspects on union cataloging of TCM ancient books， including the methods and scope， cataloging principle and rules， and working mechanism. Through specific practice and application examples， this paper expounds the value of the union cataloging ideas at the application level， such as highlighting the layout of academic disciplines， revealing the development law of the document， reflecting the academic function of inheritance of different book versions and promoting the development of ancient books industry， guiding the purchase and collection of ancient books， and guiding the excavation and utilization of ancient books. The union cataloging ideas are the concrete embodiment of the holism concept in ancient books cataloging， and lay an important foundation for the collation and especially textual research of TCM ancient books. Only by firmly upholding the union cataloging ideas can we actively and effectively promote the collation， protection and utilization of TCM ancient books in the new era， and further strengthen the coordinated innovation and shared development of libraries in the TCM industry.