This study tested the mediating role of bullying attitudes and moderating role of school connectedness in the link between sympathy and Chinese adolescents' active defending behaviors in bullying. Participants were 808 Chinese adolescents (53.3% male, M[subscript age] = 12.40, SD = 0.50). The results showed that sympathy was significantly positively correlated with active defending behaviors through bullying attitudes. In addition, school connectedness could moderate the relationship between sympathy and active defending behaviors. Specifically, the positive correlation between sympathy and active defending behaviors was stronger for adolescents with a high level of school connectedness. These findings emphasize the importance of considering complex multiple ecological level factors when examining bullying bystander.