Post-event rumination (PER) has been identified as an important maintenance of social anxiety disorder (SAD) in cognitive models. Once individuals fall into rumination, their thoughts may be rigidified which means a loss of cognitive flexibility (CF). The present study primarily focuses on reducing social anxiety (SA) through one-session targeted PER intervention of high-level social anxiety adolescents and the use of cognitive restructuring techniques. In addition, to uncover the mechanism of how PER and CF influenced SA changes, a sample of 140 Chinese high socially anxious adolescents was recruited and randomly assigned to the intervention group ( n=61) or the control group ( n=79). PER, CF, and (state) SA were measured at different times before and after the intervention. Analysis of repeated measure ANOVA results showed that PER and SA symptoms of adolescents were significantly improved after intervention, with the intervention group showing greater improvement than the control group. Furthermore, flexibility options (FO), a component of CF, fully mediated the relationship between SA and PER. Overall, the findings demonstrated that PER and SA were effectively reduced through the one-session targeted PER intervention. The study discusses the intervention mechanism, clinical implications, limitations, and future directions.