Abstract: Objective To investigate the effect of Helicobacter pylori (H.pylori) infection on dextran sulfate sulphate (DSS)-induced chronic colitis in mice, and explore its potential mechanism. Methods Chronic colitis was induced by three 5-day cycles of 2% DSS in C57BL/6J mice infected with H.pylori.Forty mice were randomly divided into 4 groups: normal control group, H.pylori infection group (HP group), DSS induced colitis group (DSS group), and DSS induced colitis with H.pylori infection group (HP+DSS group).The disease activity index (DAI), colon length and histopathologic scores of colitis in DSS group and HP+DSS group were evaluated.The degree of in filtration of CD3⁺T lymphocytes was evaluated by immunohistochemical staining, and Th17 and Treg subsets in lamina propria were detected by fiow cytometry. Results The amounts of CD3⁺T lymphocyte in filtration in colon of control group and HP group were (10.83±3.97)/HPF vs (30.67±7.37)/HPF, respectively (P＜ 0.01).Compared with the DSS group, DAI (P＜ 0.05), shortening of colon length (6.84±0.64) cm vs (8.14±0.42) cm, P＜ 0.01, histopathologic score (9.50±1.38) vs (6.83±1.72), P＜ 0.05 and in filtration of CD3⁺T cells in colon (205.17±32.28)/HPF vs (125.17±15.66)/HPF, P＜ 0.01 decreased more signi ficantly in HP+DSS group.H.pylori infection decreased Th17 subsets in lamina propria of HP+DSS group (4.56%±0.34)% vs (2.09%±0.35)%, P＜ 0.05, while it increased Treg subsets (2.01%±0.30)% vs (6.46%±0.33)%, P＜ 0.05 as compared with DSS group. Conclusion H.pylori infection has a protective effect on DSS-induced chronic colitis in mice.The immune response mediated by Th17 and Treg in colon may be involved in the process.