Objective To describe the trend of obesity prevalence in children under the new criteria of "Overweight and Obesity Screening for School-age Children and Adolescents",So as to provide clues for intervention and prevention. Methods Totally 929 students were selected as the subjects in September 2018.The trends of obesity prevalence and body mass index Z(BMI-Z) score were described from the first grade of primary school (7 years old) to the first grade of junior middle school (13 years old). Results The average age of the subjects ranged from 6.91 to 13.58 years old.Under the new criteria,the prevalence of obesity and prevalence of overweight or obesity in the first grade of primary school (seven years old) were at a peak,the prevalence of obesity was as high as 28.52% among boys,15.59% among girls.In general,the prevalence of obesity,overweight or obesity decreased with age among both boys and girls.The prevalence of obesity decreased from 28.52% to 15.55%,and the prevalence of overweight or obesity decreased from 48.83% to 39.68% among boys.The prevalence of obesity decreased from 15.59% to 8.56% and the prevalence of overweight or obesity decreased from 33.81% to 21.66% among girls.BMI-Z score showed a trend of increasing firstly and then decreasing slightly (the peak point was in the fifth grade of primary school). Conclusions Under the new criteria,the obesity situation of primary school students is severe,especially in the first grade of primary school,which is the peak period of obesity.This strengthens the urgency of childhood obesity prevention work.Primary school as an important barrier for childhood obesity intervention,should be further strengthened.