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Objective: To investigate the relationship between body mass index (BMI) and sexual development in Chinese children. Methods: A nationwide multicenter and population-based large cross-sectional study was conducted in 13 provinces, autonomous regions and municipalities of China from January 2017 to December 2018. Data on sex, age, height, weight were collected, BMI was calculated and sexual characteristics were analyzed. The subjects were divided into four groups based on age, including ages 3-<6 years, 6-<10 years, 10-<15 years and 15-<18 years. Multiple Logistic regression models were used for evaluating the associations of BMI with sexual development in children. Dichotomous Logistic regression was used to compare the differences in the distribution of early and non-early puberty among normal weight, overweight and obese groups. Curves were drawn to analyze the relationship between the percentage of early puberty and BMI distribution in girls and boys at different Tanner stages. Results: A total of 208 179 healthy children (96 471 girls and 111 708 boys) were enrolled in this study. The OR values of B2, B3 and B4+ in overweight girls were 1.72 (95 %CI : 1.56-1.89), 3.19 (95 %CI : 2.86-3.57), 7.14 (95 %CI : 6.33-8.05) and in obese girls were 2.05 (95 %CI : 1.88-2.24), 4.98 (95 %CI : 4.49-5.53), 11.21 (95 %CI : 9.98-12.59), respectively; while the OR values of G2, G3, G4+ in overweight boys were 1.27 (95 %CI : 1.17-1.38), 1.52 (95 %CI : 1.36-1.70), 1.88 (95 %CI : 1.66-2.14) and in obese boys were 1.27 (95 %CI : 1.17-1.37), 1.59 (95 %CI : 1.43-1.78), and 1.93 (95 %CI : 1.70-2.18) (compared with normal weight Tanner 1 group,all P <0.01). Analysis in different age groups found that OR values of obese girls at B2 stage and boys at G2 stage were 2.02 (95% CI : 1.06-3.86) and 2.32 (95% CI :1.05-5.12) in preschool children aged 3-<6 years, respectively (both P <0.05). And in the age group of 6-10 years, overweight girls had a 5.45-fold risk and obese girls had a 12.54-fold risk of B3 stage compared to girls with normal BMI. Compared with normal weight children, the risk of early puberty was 2.67 times higher in overweight girls, 3.63 times higher in obese girls, and 1.22 times higher in overweight boys, 1.35 times higher in obese boys (all P< 0.01). Among the children at each Tanner stages, the percentage of early puberty increased with the increase of BMI, from 5.7% (80/1 397), 16.1% (48/299), 13.8% (27/195) to 25.7% (198/769), 65.1% (209/321), 65.4% (157/240) in girls aged 8-<9, 10-<11 and 11-<12 years, and 6.6% (34/513), 18.7% (51/273), 21.6% (57/264) to 13.3% (96/722), 46.4% (140/302), 47.5% (105/221) in boys aged 9-<10, 12-<13 and 13-<14 years, respectively. Conclusions: BMI is positively correlated with sexual development in both Chinese boys and girls, and the correlation is stronger in girls. Obesity is a risk factor for precocious puberty in preschool children aged 3-<6 years, and 6-<10 years of age is a high risk period for early development in obese girls.