Objective: To analyze the relationship between body mass index(BMI) during pre pregnancy, maternal lipid levels during early pregnancy and childhood overweight and obesity, as well as the mediating role of maternal lipid levels during early pregnancy in pre pregnancy BMI and childhood overweight and obesity, providing scientific evidence for developing obesity prevention strategies in preschool children. Methods: Using data from Peking University Birth Cohort in Tongzhou (PKUBC-T) collected between June 2018 and September 2022, the study included 1 292 mother-child pairs. Participants were stratified into two groups based on children s BMI Z scores at age 3: an overweight/obesity risk group (BMI Z >1, n =173) and a non overweight/obesity risk group (BMI Z ≤1, n =1 119).Multivariate Logistic regression was conducted to analyze the associations between pre pregnancy BMI, maternal lipid levels[total cholesterol(TC),triglyceride（TG）,high density lipoprotein cholesterol(HDL-C),low density lipoprotein cholesterol(LDL-C),TC/HDL-C,TG/HDL-C,LDL-C/HDL-C] during early pregnancy and childhood overweight and obesity. The mediating effect of maternal lipid levels during early pregnancy on pre pregnancy BMI and childhood overweight and obesity was further explored. Results: There were statistically significant differences in pregnancy BMI levels, early pregnancy blood LDL-C ,TC/HDL-C,LDL-C/HDL-C levels between the overweight and obesity risk group and the non overweight and obesity risk group ( χ 2/Z =19.01, 2.48, 2.48, 2.71, all P <0.05). Multivariate Logistic regression analysis showed that pre pregnancy BMI, LDL-C, TC/HDL-C and LDL-C/HDL-C in early pregnancy were significantly associated with childhood overweight and obesity ( OR =1.09, 1.42, 1.49, 1.60, all P <0.05). LDL-C, TC/HDL-C and LDL-C/HDL-C in early pregnancy played a significant mediating role on pre pregnancy BMI and childhood obesity and the mediating effects accounted for 7.3%, 10.2%, 23.5% of the total effects, respectively (all P <0.05). Conclusions Maternal hyperlipidemia during early pregnancy partially mediated the association between pre pregnancy obesity and childhood obesity. Both pre pregnancy obesity and maternal hyperlipidemia during early pregnancy are risk factors for obesity in preschool children.