Bystanders are the most common role that adolescents play in bullying episodes, they have considerable influence on the formation of the victim's experience and the perpetrator's behavior. Based on the social-cognitive model, the current study examined the mediating role of moral disengagement in the association between callous-unemotional traits and bystander behavior and the moderating roles of moral identity and perceived social support. Participants included 2,286 Chinese adolescents aged 11-16years (49.3% boys; Mage = 13.46, SDage = 0.93). The study showed callous-unemotional traits were significantly and positively associated with bystander behavior and this relation was partially mediated by moral disengagement. Moral identity moderated the relation between callous-unemotional traits and moral disengagement as well as callous-unemotional traits and bystander behavior. Perceived social support moderated in the direct and indirect associations between callous-unemotional traits and bystander behavior via moral disengagement. The relation between callous-unemotional traits and moral disengagement and the relation between callous-unemotional traits and bystander behavior became weaker for adolescents with high perceived social support. Surprisingly, the relation between moral disengagement and bystander behavior became stronger for adolescents with a high level of perceived social support. The results supported two specific patterns of perceived social support: stress-buffering and reverse stress-buffering. The present study contributes to our understanding of the key mechanisms underlying the association between callous-unemotional traits and adolescents' bystander behavior.