Objective: To explore the relationship between socio ecological risk factors (SERF), physical activity (PA) developmental trajectories, and depressive symptoms among junior and senior high school students in Tianjin, in order to provide theoretical support for comprehensive interventions for adolescent depression. Methods: A longitudinal follow up design was adopted. In September 2022, a baseline survey was conducted using a stratified cluster random sampling method in two junior high schools and two senior high schools in Tianjin, collecting data on students basic information, SERF, PA, and depressive symptoms. Two follow up surveys were conducted in September 2023 and 2024, yielding 588 valid participants. Latent class growth analysis (LCGA) was used to identify PA developmental trajectory categories among junior and senior high school students. Logistic regression was applied to examine the associations between depressive symptoms and SERF as well as PA trajectories. Results: The detection rates of depressive symptoms among Tianjin junior and senior high school students over the three years were 26.53%, 20.24%, and 21.26 %, respectively. Depressive symptoms were positively correlated with SERF ( OR=1.04, 95%CI=1.03-1.05, P <0.05). The highest risks were observed in the individual dimension and family dimension ( OR =1.28, 1.21, both P <0.05). LCGA identified three PA trajectory groups:persistently low level (80.65%), persistently high level (4.58%), and slowly increasing group ( 14.77 %). Multivariate regression analysis showed that compared with the persistently low level PA, the slowly increasing PA significantly reduced the risk of depressive symptoms ( OR=0.44, 95%CI =0.20-0.88), while SERF still increased the risk of depressive symptoms ( OR=1.04, 95%CI =1.03-1.05) (both P <0.05). Conclusion SERF are risk factors for depression symptoms among junior and senior high school students, whereas slowly increasing PA development trajectory demonstrates a protective effect.