Adolescence is a critical period during which interactions with parents and peers play key roles in shaping mental well-being, yet there is ongoing debate regarding their relative importance. To address this gap, this research examines the relationships among communication with parents, social interactions with peers, and adolescents' mental well-being using data from 33,824 Chinese adolescents (ages 10-18 years, Mage = 13.55, 47.43% female) across three data collection waves. A random intercepts cross-lagged panel model (RI-CLPM) was combined with local structural equation modeling (LSEM) to disentangle between-person and within-person effects and examine potential age-related moderation. The results revealed significant correlations among the random intercepts of the three variables. The within-person cross-lagged effects of communication with parents on adolescents' mental well-being were significant, but not vice versa. The within-person cross-lagged effects of mental well-being on social interactions with friends were significant but not vice versa. Age moderated between-person level association, with mental well-being and peer interactions showing a U-shaped trend and the association between mental well-being and parent-adolescent communication increasing linearly across age. In conclusion, the findings suggest that parent-adolescent communication is important in predicting adolescents' mental well-being and supporting positive peer interactions across adolescent development.