Abstract：Th17/Treg imbalance plays a crucial role in the progression of ulcerative colitis （UC）. UC belongs to the category of enduring diarrhea in traditional Chinese medicine （TCM）. Its core pathogenesis is spleen and kidney deficiency and pathogen latent in the intestinal collaterals. The main pathological feature is deficiency in the origin and excess in the superficiality. The alternating attack of the disease is highly consistent with the theory of diseases induced by latent pathogens in TCM. This study illustrates the connotation of the theory of pathogen latent in the intestinal collaterals and the evolution of TCM pathogenesis of UC. At the same time， it sorts out the mechanism of Th17 cells mediating pro-inflammatory response and Treg cells maintaining immune tolerance， and clarifies the pathological process of Th17/Treg imbalance inducing intestinal inflammation and damaging intestinal mucosal barrier. On this basis， the correlation between the deficiency and excess pathogenesis of UC in TCM and Th17/Treg immune imbalance was analyzed. It was believed that healthy-qi deficiency resulting in latent pathogenic qi in remission stage was related to the deficiency of Treg function and the mild accumulation of Th17，and the transformation of pathogenic factors into heat and toxin in active stage corresponded to the excessive activation of Th17 and inflammatory storm. It is concluded that the principle of staged treatment by replenishing qi and supporting healthy qi，clearing heat and removing toxin，and activating blood to unblock collaterals is proposed. It is believed that classical prescriptions such as Shenling Baizhu Powder，Sishen Pills，Shaoyao Decoction，and Baitouweng Decoction may treat UC by regulating Th17/Treg balance and regulating the expression of inflammatory factors.