Objective: To investigate the epidemiological characteristics of premature eruption of permanent molars and its aasociation with body mass index (BMI), to provide a reference for childhood oral health promotion. Methods: A total of 861 children aged 9 to 12 years from two primary schools in Bengbu City were selected by cluster sampling method. Parental questionnaire was administered to collect socio demographic information. The eruption of second permanent molars were examined. Data was analyzed by multivariate Logistic regression model and margins command. Results: The detection rate of premature eruption of second permanent molars was 26.5%(228), 27.5% in boys and 24.9% in girls( χ 2=0.73, P =0.39). Early detection rate of second permanent molars (39.0%) was significantly higher in obese group than normal weight group (21.5%)( χ 2=21.85, P <0.01). Logistic regression analysis showed that obesity was positively correlated with the risk of premature eruption of second permanent molars( OR= 3.55 , 95%CI=2.14-5.87, P <0.01). Overweight was not associated with higher risk of premature eruption of second permanent molars( OR=1.64, 95%CI=0.95-2.81, P =0.07). Being female was associated with higher risk of premature eruption of second permanent molars compared to age matched peers( OR=2.19, 95%CI=1.42-3.39, P <0.01). Conclusion Childhood obesity is associated with higher risk for premature eruption of second permanent molars. Girls are more likely to have second permanent molar erupted in advance compared to age matched boys.