Objectives <p>Non-suicidal self-injury (NSSI) is highly prevalent during adolescence and has been linked to adverse family environments. However, little is known about the dynamic mechanisms through which harsh parenting contributes to NSSI over time. This study examined the longitudinal associations among harsh parenting, basic psychological needs frustration (BPNF), and adolescent NSSI, and tested whether BPNF mediates this relationship at both the between-person and within-person levels.</p> Methods <p>A three-wave longitudinal study was conducted among 1014 Chinese adolescents recruited from 10 middle schools in Beijing. Data were collected at three time points: February 2025 (T1), June 2025 (T2), and October 2025 (T3). Harsh parenting, BPNF, and NSSI were assessed using validated self-report measures. Cross-lagged panel models (CLPM) and random-intercept cross-lagged panel models (RI-CLPM) were estimated to distinguish stable between-person differences from within-person fluctuations over time, controlling for key demographic variables.</p> Results <p>The CLPM indicated reciprocal associations between harsh parenting and NSSI and revealed a significant indirect pathway from harsh parenting to NSSI via BPNF. After accounting for stable between-person differences, the RI-CLPM showed that within-person increases in harsh parenting predicted subsequent increases in BPNF, which in turn predicted later increases in NSSI. The longitudinal indirect effect remained significant at the within-person level.</p> Conclusion <p>These findings suggest that fluctuations in harsh parenting contribute to adolescent NSSI through increased frustration of basic psychological needs. Targeting family interactions and adolescents’ psychological need satisfaction may represent important avenues for preventing NSSI.</p>