Abstract: Objective To describe the associations of physical activity(PA)and obesity with mental health among middle school students in in Xuzhou,Jiangsu province.Methods A total of 5 730 boys and girls were selected from 10 middle schools with randomly clustering sampling and surveyed with a questionnaire and physical examination. Depressive symptoms of the students were assessed using Children's Depression Inventory(CDI)and their anxiety symptoms were assessed with the Screen for Child Anxiety Related Emotional Disorders(SCARED).Self-reported PA and other sociodemographic information were also assessed.Descriptive statistics and logistic regression model were used.Results The prevalence rate of obesity was 12.3%;the prevalence rate of depressive symptoms was 17.5%,and that of anxiety symptoms was 27.7%.Among the adolescents,self-reported rates of participating in moderate,vigorous, and insufficient PA were 13.6%,32.2%,and 63.2%,respectively.After adjusting for screen time,obesity and other potential confounders,insufficient PA was positively associated with depressive(odds ratioOR=1.18,95% confidence intervalCI=:1.02-1.37)and anxiety symptoms(OR=1.15,95% CI:1.01-1.30).Insufficient PA as a moderate factor enhanced the association between obesity and depressive symptoms in junior high school students with the OR from 1.33 to 1.50.Conclusion Insufficient PA and obesity were closely related to the prevalence of mental health among middle school students in Xuzhou.Insufficient PA moderated the association between obesity and depressive symptoms in junior high school students.