Objective To observe the interventional effects of complex probiotic powder on blood bio-chemical indices and vaginal flora in pregnant women with high risk of gestational diabetes mellitus(GDM). Methods One hundred and ten pregnant women with high risk of GDM recruited from the outpatient clinics of the obstetric department and nutrition department in this hospital were randomly divided into the probiotic intervention group (n=55) and the placebo control group (n=55).The intervention group was given 1 bag of complex probiotic powder per day,and the control group was given 1 bag of placebo powder.The colors,shapes,sizes and tastes of powders in the two groups remained the consistency.The intervention duration in this trial was 23 weeks.The questionnaire survey were conducted at baseline and at the intervention endpoint,and the blood biochemical indexes detections were performed at baseline,after 24-28 weeks of gestation,and after 34-38 weeks of gestation,respectively.In addition,the differences in the composition of the vaginal flora structure between the two groups of high risk pregnant women with GDM were detected at the intervention endpoint.Results A total of 97 cases were eventually included in the statistical analysis.Among them,there were 50 cases in the probiotic intervention group and 47 cases in the placebo control group.At the end of the 23 week intervention,there were no statistically significant differences in fasting blood glucose,red blood cells,hemoglobin,leukocytes,ferritin,total protein,albumin,glutamic transaminase,glutamic oxalate transam-inase and total bilirubin between the probiotic intervention group and placebo control group (P>0.05).In terms of vaginal flora,there was a significant difference in some indicators of α diversity between the probiotic group and placebo control group (P<0.05),and there was no significant difference in β diversity between the two groups (P>0.05).At the level of vaginal flora phyla,there were significant differences in the Firmicutes,Actinobacteria,Desulfobacterota,Deferribacterota,and δ-Myxococcota between the two groups (P<0.05).At the level of vaginal flora genera,the relative abundance of Lactobacillus spp.in the probiotic intervention group was significantly higher than that in the control group (P<0.05).Conclusion Complex probiotic pow-der supplementation may have a limited role in improving blood glucose and other related biochemical indica-tors in high-risk pregnant women with GDM.But it has a certain role in regulating the vaginal micro ecological balance in pregnant women with high risk of GDM.