<p>Previous studies indicates that helping behaviors (prosocial behavior, which benefits others, and social support, which benefits oneself) are closely associated with adolescents’ self-development. However, it is still unclear what the mutual relations and potential mechanisms are between prosocial behavior, social support, and core self-evaluation. A total of 1214 Chinese adolescents were surveyed three times, with a six-month interval between each survey (<i>M</i>_age = 15.46, <i>SD</i> = 0.71; 39.3% girls). Using several Random Intercept Cross-Lagged Models (RI-CLPMs), this study examined the relationships among these variables. The results showed reciprocal relations between prosocial behavior and core self-evaluation, between social support and core self-evaluation, and between prosocial behavior and social support. Additionally, this study identified two mediation mechanisms and serial self-enhancement mechanisms. Specifically, prosocial behavior impacted adolescents’ core self-evaluation via social support, core self-evaluation also influenced prosocial behavior through social support. Prosocial behavior and core self-evaluation could achieve self-enhancement via social support, and prosocial behavior could also be improved via core self-evaluation. These findings reveal that prosocial behavior, social support, and adolescents’ core self-evaluation can influence each other over time, emphasizing the mediating mechanism of social support between prosocial behavior and adolescents’ core self-evaluation.</p>