Abstract: Objective: To establish the BMI percentile curve reference standard and the critical value of overweight and obesity for children aged 3~6 years in China. Methods: 1) The physical fitness data of 101 861 children from 31 provinces in 2010 and 2014 were used for analysis. The percentile curve of BMI was constructed by using GAMLSS model with four parameters: Median,standard deviation, kurtosis and skewness. 2) The percentile integration method was used to establish the borderline between overweight and obesity in children aged 3~6 years in China by connecting with the borderline for 7-year old children recommended by WGOC. Results: 1) The optimal model for male and female children were all BCTo distribution models with excellent results of goodness-fit-test. In this model, the power conversion coefficients of independent variable were 0.601 7 and 0.250 1, respectively;and the total degrees of freedom of the parameters were 18.11 and 16.26, respectively. Based on the 2018—2019 physical fitness data, the regression test showed that the difference between the percentage under the standard percentile curve and the actual percentage in each age group was within 2%, which showed the model was effective. Finally, a reference standard for BMI percentile curves of 3~6 years old children in China with excellent smoothness was obtained. 2) The percentile curves of cut-off values of overweight and obesity in male children were C79.6 and C91.5, respectively and that in female children were C85.4 and C96.4, respectively. The critical values of overweight and obesity presented in the forms of table and formula were obtained.Conclusions: The reference standard of BMI percentile curve and the cut-off values of overweight and obesity established in this study were widely representative and high effective. The results of this study provided references for the evaluation of growth and development and the identification of overweight and obesity in 3~6 years Chinese children.