<p>Pro-environmental volunteering is a key way in which university students contribute to climate action, yet little is known about how such behavior diffuses through peer networks, whether peer influence outweighs homophilous selection, and how these processes unfold within digitally mediated Chinese university contexts. To address these gaps, this study examined dual social-network mechanisms, digital socialization, and social self-efficacy in the spread of pro-environmental volunteering during the first year at university. Data came from 1,151 first-year students (M _age = 17.91, SD = 0.78; 51.9% female) nested in complete friendship networks across six Chinese universities and surveyed twice over six months. Longitudinal stochastic actor-oriented models were used to disentangle homophilous selection from peer influence, and regression-based mediation and moderation analyses tested social self-efficacy as a psychological pathway and social media usage as a contextual moderator. Peer influence on pro-environmental volunteering was substantially stronger than homophilous selection (influence–selection ratio = 2.64, 95% CI [1.82, 3.91]), indicating that behavior spread primarily through post-relationship socialization rather than friendship choice. Social media usage did not significantly alter the tendency to befriend behaviorally similar peers, but higher overall usage selectively amplified peer influence, with significant effects emerging only above moderate usage levels. Social self-efficacy partially mediated the longitudinal association between peers’ volunteering and individual volunteering, accounting for 54% of the total effect. These findings integrate network co-evolution, digital socialization, and social-cognitive perspectives to clarify how environmentally significant behaviors disseminate within youth peer networks.</p>