<p>To investigate the potentially unique effects of mindfulness practice on social anxiety in college-bound high school seniors and explore the psychological mechanisms by which it works, this study compared the mindfulness-based intervention (MBI) group with the wait-list control (WLC) group. A total of 66 students who had just graduated from high school and were about to enter college were randomly assigned to the MBI group (<i>n</i> = 33) or WLC group (<i>n</i> = 33) and conducted self-reported mindfulness, intolerance of uncertainty, emotion dysregulation, and social anxiety at baseline and post-intervention. Repeated measurement analysis of variance demonstrated that variables varied significantly over group (MBI group and WLC group) × time (pre-test and post-test). Based on structural equation models (SEM), it has been demonstrated the change in the degrees of mindfulness, intolerance of uncertainty, and emotion dysregulation plays a partial mediating role in the process of alleviating the level of social anxiety with mindfulness intervention. MBI can not only directly affect social anxiety, but also indirectly affect the level of social anxiety through three paths. In conclusion, MBI can effectively help high school leavers improve their level of mindfulness, reduce unbearable uncertainty and emotion dysregulation. Moreover, MBI can alleviate social anxiety in high school graduates preparing for university by improving their level of mindfulness and reducing intolerance of uncertainty and emotion dysregulation.</p>