Abstract: Based on equity distribution theory and the General Aggression Model， the study examined whether parental differential treatment could compromise adolescents’ belief in a just world， further leading to offline bullying and online bullying. Participants were 582 Chinese middle school students （309 girls and 273 boys； Mage = 14.39， SD = 1.01）， who completed questionnaires on parental differential treatment， offline bullying， online bullying， and belief in a just world. Results suggested that： （1） parental differential treatment was positively associated with offline bullying and online bullying and negatively associated with adolescents' belief in a just world； belief in a just world was negatively associated with offline bullying and online bullying； （2） parental differential treatment positively predicted offline bullying and online bullying； and （3） belief in a just world mediated the relationship between parental differential treatment and offline bullying and online bullying. This study aims to clarify the impact of parental differential treatment on adolescents’ various forms of bullying behavior and its mechanisms， with the intention of providing effective guidance and recommendations for parenting practices in multi-child families.