Previous research has shown that secure attachment promotes prosocial behavior. However, the mechanism underlying the link between attachment and prosocial behavior has received relatively little attention. The current study examined the mechanism underlying the link between attachment and prosocial behavior by specifically focusing on the potential mediating role of moral disengagement and whether this mediation effect is moderated by moral identity. Self-report questionnaires designed to measure attachment, moral identity, moral disengagement, and prosocial behavior were administered to Chinese students (n = 395; 250 females; M-age = 23.38 years). Results showed that secure attachment (maternal attachment, paternal attachment, and peer attachment) was associated with prosocial behavior. In addition, moral disengagement was found to mediate the association between secure attachment and prosocial behavior. Moreover, the mediation effect was moderated by moral identity; specifically, the mediation effect was stronger for individuals with a high level of moral identity than their low-level moral identity counterparts. The present results suggest that morality could explain the association between attachment and prosocial behaviors.