Abstract: OBJECTIVE To explore the influence of Lactobacillus on intrauterine infection and serum helper T cell 17/regulatory T cell (Th17/Treg) cytokines during pregnancy. METHODS From Apr 2016 to May 2017, a total of 92 pregnant women with second trimester of pregnancy who determined to give birth in Shijiazhuang Fourth Hospital were enrolled in the study and randomly divided into the treatment group and the control group. The treatment group was treated with vaginal administration of lactic acid bacteria capsule, while the control group was not treated with any intervention. The cleanliness of vaginal secretions was compared between the two groups of pregnant women after the intervention, the levels of Lactobacillus, Th17/Treg and serum cytokines were compared before and after the intervention, the incidence of intrauterine infection, status of rupture of membranes and newborn Agar score were observed and compared. RESULTS The incidence rate of intrauterine infection of the treatment group was significantly lower than that of the control group, the percentage of the pregnant women whose newborn Apgar score was no more than 7 points was significantly lower in the treatment group than in the control group (P<0.05). There was no significant difference in the artificial rupture of fetal membranes or full-term rupture of membranes. There were no significant differences in the cleanliness of vaginal secretions, Lactobacillus level, Th17/Treg, interleukin-17 (IL-17) level, interleukin-23 (IL-23) level, interferon-gamma (INF-γ) level and tumor necrosis factor-alpha (TNF-α) level between the two groups of pregnant women before the intervention, while the above indexes of the treatment group were significantly improved after the intervention(P<0.05), they did not change significantly in the control group. CONCLUSION The vaginal administration of Lactobacillus may improve the vaginal cleanliness of the pregnant women, reduce the incidence of intrauterine infection and effectively improve the Th17/Treg as well as the levels of cytokines.