Abstract： Yin deficiency syndrome is a clinical syndrome that both the Danxi School and the Warm Supplementation School focus on and explore. By sorting out the evolution of the concept of "yin deficiency" from the Danxi School to the Warm Supplementation School， it is believed that the evolution of the connotation of this concept highlights the different understandings of the specific reference to "yin" in different eras，as well as the mechanisms，changes，and outcomes of the occurrence of "yin deficiency". ZHU Danxi's understanding of "yin deficiency" included various connotations such as yin-blood deficiency， yin-essence deficiency， kidney deficiency， and insufficient kidney water； his private disciple YU Tuan believed that this "yin deficiency" should be divided into yin deficiency in qi， yin deficiency in blood， and genuine yin deficiency in the kidney channel；WANG Ji believed that this "yin deficiency" is "ying deficiency"， which included two aspects：ying qi deficiency and ying blood deficiency. XUE Ji，a scholar of the Warm Supplementation School in the Ming Dynasty，believed that spleen earth was crucial in the development， transformation，and treatment of yin deficiency，and proposed the views that "yin deficiency" was "spleen deficiency" and "deficiency of three yin channels of the foot". In the late Ming Dynasty，ZHANG Jingyue and ZHAO Xianke developed the theory of gate of vitality，emphasizing that the "genuine yin" of the human body was hidden in the kidney gate of vitality，which was related to the innate nature of the human body；they believed that the essence contained in it could transform into original qi，including both water and fire，and proposed that there were two types of yin deficiency. The development of the connotation of "Yin deficiency" in the Warm Supplementation School is aimed at correcting the bitter and cold bias caused by the Danxi School， and also promotes the deepening of the yin and yang theory and the viscera manifestation theory.