Objective: To investigate the association between moderate to vigorous physical activity (MVPA) and sedentary behavior with the risk of overweight/obesity in children,and to provide a reference for health life style of primary school students. Methods: From November to December 2018, a cross sectional survey of 949 school children aged 9-12 years in Guangdong Province was conducted. Children were classified as normal weight or overweight/obesity based on body mass index(BMI). Participants were advised to wear a wristwatch type Gene Active accelerometer for 24 hours to record MVPA and sedentary time. Children s screen time (ST) was provided by parents. Poisson regression was used to analyze the independent associations between MVPA, sedentary time, screen time and overweight/obesity risk. Results: About 20.8%(197) of the children were overweight/obesity. The percentage of children who met the MVPA recommendation was 22.1%(210), and the amount of sedentary time (excluding sleep) during the school day was 60.8%(577), or as high as 8.5 hours per day. Only 12.3%(117) of children have less than two hours of screen time per day. Compared with Q 4, pupils with MVPA of Q 1 had a 4.38 higher risk of overweight/obesity compared with normal pupils ( P <0.01), no similar associations were observed for sedentary time. Compared with pupils who met the recommended MVPA, pupils with lower MVPA had an increased risk of overweight/obesity by 2.54 times(95% CI =1.59-3.77). Pupils with more than 2 hours per day had an increased risk of overweight/obesity by 1.87 times(95% CI =1.21-3.02) compared with pupils with less than 2 hours per day. Conclusion The risk of overweight/obesity decreases with the increase of MVPA. The proportion of students meeting the MVPA recommendation is relatively low, the proportion exceeding screen time recommendation is high.