Objective: To explore the individual and joint effects of outdoor activities and different types of sedentary behavior on adolescent internalizing behaviors, so as to provide a basis for the prevention and intervention of such behaviors. Methods: From July to December 2023, a stratified cluster random sampling method was used to select 1 691 primary and secondary school students in grades 5-12 from four primary and secondary schools in Baise City. Anonymous surveys were conducted by using the Outdoor Activities Questionnaire,Adolescent Sedentary Activity Questionnaire, and Youth Self Report. The Chi square test and Logistic regression analyses were used for statistical analysis. Results: The detection rate of internalizing behavior among adolescents was 13.13%, and the detection rate of internalizing behaviors in girls (15.53%) was significantly higher than that in boys ( 10.77 %) ( χ 2=8.39, P <0.01). Logistic regression analysis showed that outdoor activities ≥1 h/d ( OR =0.56), screen based entertainment sedentary behavior < 2 h/d ( OR =0.58), and cultural sedentary behavior < 2 h/d ( OR =0.55) significantly reduced the risk of internalizing behaviors (all P <0.05). Logistic regression analysis of interaction effects revealed that the risks of internalizing behaviors were lower in the groups with "outdoor activities ≥1 h/d + screen based entertainment sedentary behavior <2 h/d" ( OR = 0.27) and "outdoor activities ≥1 h/d + cultural sedentary behavior <2 h/d" ( OR =0.30)(both P <0.01). Conclusions Outdoor activities can mitigate the adverse effects of sedentary behaviors on adolescents internalizing behaviors. Reducing screen time and leveraging the synergistic effects of "outdoor + cultural" activities may help reduce the risk of internalizing behaviors among primary and secondary school students.