<p>Social exclusion deprives junior high school students of positive psychological and social resources, but its relationship with flourishing and its internal mechanism of action have not yet been revealed. This study aims to deeply explore how social exclusion affects junior high school students’ flourishing, based on theories such as the telic theory of subjective well-being and the temporal need-threat model. This study seeks to clarify the relationship between social exclusion and flourishing and analyze the roles of potential mediating and moderating variables. The questionnaires—including the Ostracism Experience Scale for Adolescents, the Self-Regulation Fatigue Scale, the Self-Compassion Scale, and the Flourishing Scale—were administered to 700 junior high school students in China. The results of the study revealed that (1) social exclusion was negatively related to flourishing; (2) ego depletion mediated the effect of social exclusion on flourishing; (3) this direct pathway between social exclusion and flourishing was moderated by self-compassion. Specifically, social exclusion negatively affected flourishing less for junior high school students with high levels of self-compassion than it did for junior high school students with low levels of self-compassion. This suggests that social exclusion not only directly affects junior high school students’ flourishing but also indirectly affects junior high school students’ flourishing through ego depletion and that self-compassion moderates the direct path of the mediation model. The results reveal the mechanism and boundary conditions of the effect of social exclusion on junior high school students’ flourishing, which is instructive for improving it.</p>