Abstract: Objective  To observe the effect and dynamic change trend of Schistosoma japonicum egg excretory-secretory antigen (ESA) on the proportion of regulatory T cells (Tregs) in the spleen of mice, and to preliminarily investigate its immunomodulatory effect on the mice. Methods  A rabbit model of Schistosoma japonicum infection was established, and then the eggs were isolated and purified from the liver tissues to prepare ESA of Schistosoma japonicum. Forty SPF-grade male Kunming mice were randomly divided into an experimental group and a control group (n=20 in each group). Mice in the experimental group were treated with Schistosoma japonicum ESA, and those in the control group were intervened with PBS for 4 consecutive weeks. Before immunization and at 4, 8 and 12 weeks after immunization, flow cytometry was used to detect the percentage of CD4+CD25+Foxp3+Tregs in CD4+T cells in the spleen of mice. The immune response was evaluated in combination with changes in body weight and spleen index of the mice. Results  At 8 and 12 weeks after ESA immunization, the body weight of mice was generally lower in the experimental group than in the control group [(40.18 g±0.56 g, 41.71 g±0.54 g) vs. (42.18 g±0.26 g, 44.71 g±0.69 g), respectively]. The difference was statistically significant (t=7.116, 7.661, both P<0.01). At 4, 8, and 12 weeks following immunization, the spleen indexes of mice in the experimental group were higher than those in the control group [(3.80 mg/g±0.04 mg/g, 4.16 mg/g±0.02 mg/g, 4.31 mg/g±0.03 mg/g) vs. (3.57 mg/g± 0.01 mg/g, 3.58 mg/g±0.01 mg/g, 3.59 mg/g±0.02 mg/g), respectively], and the difference was statistically significant (t=-11.309, -58.881, -48.882, all P<0.01). Meanwhile, the proportions of Tregs in CD4+T cells in the spleen of mice in the experimental group at 4, 8, and 12 weeks after immunization were 20.15%±1.01%, 24.92%±1.06% and 30.29%±1.06%, respectively, which were higher than those in the control group at the corresponding time points (14.27%±0.66%, 15.40%±0.86% and 16.56%±0.86%), with significant statistical differences (t=-10.343, -15.637, -22.494, respectively, all P<0.01). Moreover, the proportion gradually increased with the extension of immunization time. Conclusion  ESA can induce the differentiation of CD4+T cells into Tregs in the spleen of mice, and this effect is positively correlated with the duration of immune intervention.