Object: To investigate relationship of the body mass index (BMI) before pregnancy, parity, with the pathogenesis of gestational diabetes mellitus (GDM). Methods: A total of 262 cases of patients with GDM who received treatment in the nutrition outpatient department from February 2014 to December 2014 were enrolled in this study, they taked up glucose tolerance test (OGTT), in accordance with WTO standards. The 262 cases of full-term pregnancy primiparous women in pregnancy between 24 and 28 weeks were given normal 75 g oral glucose tolerance test, and recorded the clinical data, such as age, gestational age, educational level, pregnancy of body mass index (BMI) and parity. According to the points of body mass index (BMI) before pregnancy, they were divided into 4 groups: BMI <18.5 kg/m2 for underweight group; BMI 18.5~23.0 kg/m2 into normal weight group; BMI 23.1~25.0 kg/m2 for overweight group; BMI >25.0 kg/m2 for obesity group. SPSS l8.0 statistical software of data was used to do descriptive analysis, two sample t test and non-conditional logistic regression analysis, analyzing the relationship among pre-pregnancy body mass index (BMI), parity and gestational diabetes. Results: A total of 4 groups of OGTT results showed that pregnancy overweight or obese pregnant women pregnancy risk of diabetes is higher than that of normal and lean body mass group; non-conditional logistic regression analysis showed that, the factors affected by GDM followed by: parity, body mass index, age, culture degree. The influence factors of regression coefficients were: 2.372, 1.075, 0.516, −0.209, which displays that body mass index and parity were two major risk factors for pregnant women suffer from gestational diabetes. Conclusion: Pre-pregnancy overweight or obese pregnant women who had more parity are at a higher risk of gestational diabetes mellitus in pregnancy.