BACKGROUND: Research has extensively explored the direct and indirect relationship between bullying victimization and the mental well-being of children with underlying mechanisms through cross-sectional data. However, limited knowledge exists regarding the causal inference and potential underlying mechanisms in this context. OBJECTIVE: This study delves into these effects among Chinese children, investigating the mediating roles of social withdrawal and the moderating role of approach coping strategies using a two-wave longitudinal design. PARTICIPANTS AND SETTING: This study collected two-wave data with a six-month interval from a cohort of 863 Chinese children. 49.7 % were identified as female and 50.3 % of samples were males, with an average age of 14.01 at Wave 1. Following the acquisition of informed consent from their legal guardians, all students voluntarily completed the self-report questionnaires. METHODS: The study utilized R software to conduct data analysis. Half-longitudinal mediation and moderation models were adopted to examine the effects on mental well-being. Multi-group path analysis was used to examine the group differences in left-behind status. RESULTS: The research results indicated that bullying victimization exhibited both direct (β = -0.085, p <
0.01) and indirect (β = -0.017, p <
0.05, 95 % CI: [-0.030, -0.002]) effects on diminished mental well-being, mediated by social withdrawal. In addition, higher levels of approach coping strategies act as a buffer in the adverse effect of bullying victimization on children's mental well-being (β = -0.054, p <
0.05). Furthermore, the study highlighted that the left-behind status significantly moderated the relationship between peer victimization and social withdrawal. Interestingly, this relationship was notably significant in non-left-behind children rather than their left-behind counterparts (β CONCLUSION: These findings enhance our comprehension of the intricate interplay between victimization and well-being, offering crucial insights for policymakers and social workers to craft targeted interventions. By developing a deeper understanding of how victimization influences psychological well-being, tailored strategies can be formulated to effectively support children impacted by such experiences.