In the present study, we tested the mediating role of moral disengagement in the effect of childhood maltreatment on bullying and defending. We also examined whether the mediating effect of moral disengagement differed between girls and boys, early and middle adolescents, as well as only and non-only children. Five hundred and fifty-two Chinese adolescents participated in this study. The results indicated that the prevalence rates of physical abuse, physical neglect, emotional abuse, and emotional neglect were 4.0%, 40.0%, 10.5%, and 27.9%, respectively. Path analysis showed that moral disengagement partially mediated the relation between childhood maltreatment and bullying, but did not mediate the relation between childhood maltreatment and defending. Moreover, multi-group analyses indicated that no significant path in the final model differed by gender. However, early adolescents exposed to high levels of childhood maltreatment were more likely to engage in bullying and less likely to engage in defending than middle adolescents, and early adolescents with high levels of moral disengagement were more likely to engage in bullying than middle adolescents. Compared to non-only children, only children exposed to high levels of childhood maltreatment were more likely to score higher on moral disengagement and less likely to engage in defending.