Objective: To explore the therapeutic effect and mechanism of Weishu capsule (WSC), Open image in new window in treating precancerous lesions of chronic atrophic gastritis (CAG).Methods: Seventy-five cases of precancerous lesions of CAG were distributed to two groups randomly, the WSC group (45 patients) treated with WSC taken orally and the Weining granule (WNG) group (30 patients) treated with Weining granule orally. The treatment course of both groups were 6 months. The clinical and pathological effects and the changes in scores of atrophy, intestinal metaplasia (IM) and its subgroups, dysplasia, carcinoembryonic antigen and proliferating cell nuclear antigen expression were observed before and after treatment.Results: The total effective rate was 91% and the effective rate of atrophy, IM, dysplasia in the WSC group were 64%, 67% and 65% respectively, and the respective scores were lowered significantly after treatment. Compared with the WNG group, the difference between the two groups was statistically significant (P < 0.01,P < 0.05). Immunohistochemical test showed that WSC could inhibit the subgroups of IM, circinoembryonic antigen and proliferating cell nuclear antigen expression significantly (P < 0.01).Conclusions: WSC could significantly improve the clinical and pathological changes in the precancerous lesions of CAG. The mechanism might be related to the inducing and promoting effect of WSC on the differentiation and maturity of IM cells and dysplasia cells. WSC could also inhibit and correct the abnormal proliferation of cells.