In order to explore the relationship and mechanism of action between just-world beliefs and cyber moral misbehavior, a moderated mediation model was constructed to explore the mediating role of online moral emotions and the moderating role of gender. The Just World Beliefs Scale, the College Students’ Cyber morality Scale, and the College Students’ Cyber moral Misbehavior Scale were used to survey 640 college students, and the results found that (1) Just World Beliefs negatively and significantly predicted Cyber moral Misbehavior; (2) Cyber moral Emotions partially mediated between Just World Beliefs and Cyber moral Misbehavior; and (3) gender played a moderating role between Cyber moral Emotions and Cyber moral Misbehavior, with males more behavior; (3) Gender plays a moderating role in the relationship between cyber moral emotions and cyber moral misbehavior, and males are more likely to be influenced by cyber moral emotions and make choices of cyber moral misbehavior.