Abstract: Objective To investigate the preventive and therapeutic effects of high dose vitamin D(VD) during gestation on gestational diabetes mellitus in obese women. Methods One hundred and eighteen pregnant women were randomly divided into two groups:the experimental group and the control group,59 cases in every group.The experimental group were given vitamin D 30 million IU intramuscular injection in the pregnant 16-20 weeks.In the control group,VD drops capsule(400 IU) was orally administered once a day for 16-20 weeks.The clinical data of 25(OH)D₃,PTH,Ca,P,HbA1c and BMI and the improvement of vitamin D deficiency were compared between the two groups. Results After intervention,the levels of 25(OH)D₃ and serum Ca in test group were significantly higher than those before treatment,PTH concentration was significantly decreased(P<0.01).In the trial group,the ratio of patients with moderate to severe vitamin D deficiency was significantly lower after treatment(P<0.01).The incidence of GDM in pregnant women was significantly lower than that in the control group(P<0.05),and the incidence of GDM in pregnant women was significantly lower than that in the control group(P<0.01).The cesarean section was significantly lower in the test group than in the control group(P<0.01).The incidence of neonatal stillbirth,neonatal asphyxia,premature infants and macrosomia in the experimental group was significantly lower than that in the control group(P<0.05). Conclusion The single dose can effectively improve the VD deficiency state of obese pregnant women,and can effectively reduce the incidence of GDM of obese pregnant women,reduce the rate of cesarean section,reduce the risk of adverse pregnancy outcome.