Although peer support is documented as a predictor of adolescents' prosocial behavior, the influencing mechanism of peer support on prosocial behavior among adolescents from low-income families has not been fully researched. The current research conducted two studies to examine the influencing mechanism of peer support on prosocial behavior among adolescents from low-income families. Study 1 (N = 579) investigated the influencing mechanism of peer support on prosocial behavior with a longitudinal dataset. Study 2 (N = 228) investigated the effect of peer support priming on money donation with an experimental approach. The results of Study 1 and Study 2 consistently demonstrated that communal social goals mediated the association between peer support and prosocial behavior among adolescents from low-income families, and cumulative family risk moderated this indirect link. The positive effect of peer support on prosocial behavior through communal social goals might be weakened if adolescents experience more risk factors in the family context.