The importance of prosocial behavior in helping migrant youths overcome challenges associated with relocating to urban settings has been emphasized. However, the trajectory of prosocial behavior in this population and the factors influencing it remain unclear. This study aimed to investigate the influence of cumulative family risk on the growth trajectory of prosocial behavior among Chinese rural-to-urban migrant adolescents from 7th grade to 8th grade. Additionally, it delved into the moderating role of school belonging in the relationship between cumulative family risk and prosocial development. The participants in this study included rural-to-urban migrant adolescents (Mean age = 13.52, SD = 1.23, N = 844) and their parents. Outcomes from latent growth curve modeling revealed that migrant adolescents displayed intra-individual increases in prosocial behavior during the transition from 7th grade to 8th grade, while cumulative family risk had a negative influence on these changes. Nevertheless, the adverse impact of cumulative family risk on prosocial development was alleviated for adolescents who reported a heightened sense of belonging within the school environment. These findings underscore the protective role of school belonging in the prosocial development of migrant adolescents, especially for those facing elevated cumulative family risk.