Objective To study the mechanism of Yi medicine Yiyuanhuafu Decoction regulating T cell subsets in the treatment of ulcerative colitis, and provide scientific basis for clinical application of Yiyuanhuafu Decoction. Methods Thirty-two healthy female C57BL/6 mice were randomly divided into four groups: vehicle group, model group, medium dose of Yiyuanhuafu Decoction group(4.05 g·kg-1) and high dose of Yiyuanhuafu Decoction group(8.1 g·kg-1). Except for the vehicle group, other models of ulcerative colitis were established by the way of free drinking water 3% sodium dextran sulfate (DSS) which replace every 2 days. From the first day of drinking DSS, the mice were given the corresponding dose by gavage in each group, once a day for 8 days. Diseases activity index (DAI) and hematoxylin-eosin staining （HE） were used to evaluate the symptoms and morphological changes of ulcerative colitis, respectively. Real-time quantitative polymerase chain reaction(q-PCR) assay was used to detect the transcriptional expression of T cell subsets related factors in lymphocytes. The expression of T cell subsets Th1、Th17 and regulatory T cell（Treg） was detected by flow cytometry. Results Compared with the vehicle group, the body mass and colon mass of the pathological model group was significantly decreased, and the colon length was significantly shortened (P<0.05 or P<0.01). Compared with the pathological model group, Yiyuanhuafu Decoction group significantly increased body weight and colon length (P<0.05 or P<0.01), and improved the degree of colon pathological injury. The results of flow cytometry showed that Yiyuanhuafu Decoction significantly down-regulated the expression of γδTCR（P<0.05）, and inhibited the proportion and absolute number of Th1 cells (CD4+ IFN-γ+ T cells) and Th17 cells (CD4+ IL-17+ T cells) (P<0.01), but had no significant effect on the expression of Treg cells(CD4+ Foxp3+ regulatory T). The results of fluorescence q-PCR showed that Yiyuanhuafu Decoction significantly down-regulated the expression of Th17 cell-related factor IL-17A and transcription factor RORγτ mRNA in UC mice（P<0.01）. Conclusion Yi medicine Yiyuanhuafu Decoction may alleviate ulcerative colitis by inhibiting the inflammatory response mediated by Th17 cells.