<p>Previous longitudinal studies have explored the developmental mechanisms of non-suicidal self-injury (NSSI) and suicidal ideation (SI) in adolescents and primarily focused on variable levels rather than trajectories (e.g., intercepts and slopes). The present study aimed to explore the developmental trajectories of NSSI and SI among Chinese adolescents, emphasizing the predictive roles of fear of negative evaluations (FNE) and thwarted belongingness (TB) on these trajectories from an interpersonal perspective. The latent growth model was incorporated to construct a mediation model of FNE, TB, NSSI, and SI. A total of 515 adolescents (<i>M</i>_age = 12.309, <i>SD</i> = 0.807; 49.3% girls) completed questionnaires on 4 waves, 6&#xa0;months apart. The results showed that: (1) FNE showed a curvilinear growth trajectory, with a progressively smaller growth rate; TB, NSSI, and SI all showed a linearly decreasing trajectory; (2) in the relationship between FNE and NSSI, TB’s intercept mediated the relationship from FNE’s intercept to NSSI’s; (3) in the relationship between FNE and SI, TB’s intercept mediated the relationship from FNE’s intercept to SI’s intercept, as well as to SI’s slope. Meanwhile, TB’s slope mediated the relationship from FNE’s slope to SI’s. This study reveals the dynamic developmental trajectories of FNE, TB, NSSI, and SI in Chinese adolescents. Notably, TB plays a different mediating role between FNE, NSSI, and SI, highlighting its critical role in the development of these behaviors. Early interventions targeting FNE and enhanced belongingness may help prevent or reduce NSSI and SI in adolescents.</p>