<p>Social anxiety is increasingly prevalent among university students and is associated with numerous negative outcomes. Stressful life events have been identified as a key risk factor for social anxiety. However, the underlying within-person processes and mechanisms remain poorly understood. The present study examined the longitudinal association between stressful life events and social anxiety, as well as the mediating role of rumination, while disentangling between-person and within-person effects using Random Intercept Cross-Lagged Panel Models (RI-CLPMs). The sample consisted of 313 university students (<i>M</i> = 19.56, <i>SD</i> = 0.76, 13.74% male), who completed a three-wave survey at six-month intervals. At the between-person level, stressful life events, rumination, and social anxiety were significantly positively correlated. At the within-person level, increases in stressful life events predicted subsequent increases in rumination, which in turn predicted elevated social anxiety over time. No evidence of reverse effects was observed. These findings enhance understanding of the complex interplay among stressful life events, rumination, and social anxiety, highlighting stressful life events as a primary antecedent of social anxiety and rumination as a critical mediating mechanism.</p>