Background: Relational victimization (RV) is a significant threat to adolescent mental health and social adaptation in China. However, its developmental characteristics during the school transition period are poorly understood. Finkelhor's developmental victimology framework provides a comprehensive lens to explore factors influencing RV's development. This study examined the trajectory of RV and the impact of gender, childhood maltreatment, internalizing problems, and family socioeconomic status. Methods: We recruited 762 students from a high school in Sichuan Province, China, to complete three questionnaires in June 2020, December 2020, and June 2021. After excluding participants who did not fully complete all three surveys or whose data were disqualified, we obtained a final longitudinal sample of 605 participants. All participants were high school freshmen, with a mean age of 15.89 years (SD = 0.59) at the time of the final survey; participants included 264 boys. We used this data to construct a conditional growth model that incorporated both time-varying and time-invariant covariates. Results: The results revealed that the RV of Chinese mid-adolescents follows a linear growth trend. Childhood maltreatment was found to be a predictor of the initial level of RV, while socioeconomic status predicted the rate of RV's growth. Internalizing problems were found to influence the development trajectory of RV. Conclusion: The findings highlighted the increasing trend of RV during school transition and the roles of childhood maltreatment, socioeconomic status, and internalizing problems in shaping RV's development. This study extended the developmental victimology framework and offers valuable insights for interventions targeting adolescents' RV.