Abstract: Objective To compare the bone mineral density(BMD) of lower limbs in children age 2-6 years old with overweight,simple obesity and low body mass index(BMI),and to explore the effects of obesity and overweight on bone strength.Methods From March 2017 to January 2018,a total of 436 children who underwent physical examination and ultrasonic bone mineral density examination in kindergarten in our hospital were selected as the subjects of the study.The subjects were divided into four groups according to BMI,including 118 healthy children in the control group,101 children in the overweight group,100 children in the obesity group and 117 in the low BMI group.The detection value(Z value) of BMD in four groups of children was compared by ANOVA.The effects of BMI,sex,height and weight on BMD in four groups of children were further analyzed by single factor χ² test and Logistic regression analysis.Results There was no statistically significant difference in the Z values between the overweight group and the control group,the low BMI group and the control group,the low BMI group and the overweight group,respectively(P>0.05),while the Z value of the obesity group was significantly lower than that of the control group,the overweight group and the low BMI group,the difference was statistically significant(P<0.05);Among the four factors of BMI,sex,height and weight,there was significant correlation between BMI,height and BMD t(P<0.05),and they were the risk factors for BMD decline(OR> 1,P<0.05).Conclusion Compared with normal healthy children,there was no significant difference in bone mineral density between the overweight children and the low BMI children,but the bone mineral density decreased obviously in the obese children.Higher BMI and higher height may be the risk factors for BMD.