Objective: To explore correlations between physical activity (PA), sedentary behavior (SB) and physical posture disorders in primary school students, and to provide reference for the prevention and correction of adverse body posture in this group. Methods: From September to December 2020, 206 children from 4 primary schools in Chaoyang District of Beijing were selected by multistage stratified cluster random sampling method. Body posture and PA were assessed using a body posture tester and a triaxial accelerometer. Children s Leisure Activity Questionnaire was used to investigate SB. Results: The detection rate of abnormal body posture in primary school students was 76.70%. PA showed positive assocation with high and low shoulder, X/O leg, pelvis forward, spine curvature and comprehensive evaluation ( β =0.17, 0.21, 0.19, 0.24, 0.19, P <0.05). SB had significant negative linear correlation with high and low shoulder, X/O leg,pelvis forward, spine curvature and comprehensive evaluation ( β = -0.24 , -0.22, -0.36, -0.24, -0.27, P <0.05). In the combination analysis, children with high PA plus low SB ( OR=2.12, 95%CI= 1.46-3.37, P <0.05) and children with low PA plus high SB had the highest and lowest advantages of qualified rate of body posture assessment,respectively. Compare with low SB and low PA( OR=1.71, 95%CI=1.41-2.74, P <0.05), high PA and high SB ( OR=1.85, 95%CI=1.38-2.86, P <0.05) had higher qualified rate of body posture assessment. Conclusion Physical activity shows positive effects on physical posture while sedentary behavior shows negative effects on physical posture. The combination of PA and SB has a counteracting or synergistic interaction effect.