Objective To explore the therapeutic effect and mechanism of Weichang’an Pill on dextran sodium sulfate（DSS）-induced ulcerative colitis in mice.Methods Sixty SPF C57 BL/6 mice were used.Except for normal control group of mice,the other mice were given 2.5% DSS aqueous solution with free consumption for 7 days to prepare ulcerative colitis models.After model evaluation,they were randomly divided into model group,Mesalazine group,Weichang’an Pill high-dose group,medium-dose group,low-dose group.The Weichang’an pill low,medium,and high dose groups were given 0.015,0.03,and 0.06 g/（kg·d）of Weichang’an pill suspension respectively by intragastric administration 0.5 mL,once a day; the mesalazine group was given 0.52 g/（kg·d）mesalazine suspension by intragastric administration of 0.5 mL once a day; the normal control group and model group were orally administered with equal volume of pure water,all for 10 consecutive days.The disease activity index（DAI）score and colon length of the mice in each group were compared,and the pathological and morphological changes of the colon tissue of the mice were observed with HE staining.The percentage of Treg,Th17,CD4,+,and CD8,+, lymphocyte in the spleen tissue was detected by flow cytometry.Serum cytokines TNF-α,IL-6,IL-8,IL-10 and CRP were detected by ELISA.Results During the drug intervention,the body weight of the mice in each group was increased day by day,and the symptoms such as diarrhea,hematochezia,and weight loss were alleviated day by day.Compared with the model group,the above symptoms could be alleviated more rapidly in the mesalazine group and Weichang’an Pill high-dose group.DAI scores of both two groups were reduced day by day（P,<,0.05）.After drug intervention,compared with the model group,the mice in the high-dose Weichang’an Pill group and mesalazine group had longer colons（P,<,0.05）,and the congestion of colon tissue,interstitial edema,and inflammatory cell infiltration were milder.Compared with the model group,the percentage of CD4,+,CD8,+,and Treg cells and serum IL-10 levels in the high-dose group of Weichang’an Pill and mesalazine were higher（P,<,0.05）; the percentage of Th17 cells and serum TNF-α,IL-6,IL-8,CRP level was lower（P,<,0.05）.Conclusion Weichang’an Pill can alleviate the intestinal mucosal damage induced by DSS in mice.The mechanism may be related to the restoration of Treg/Th17 immune balance and the regulation of related cytokines.