<p>Problematic mobile social media use (PMSMU) has emerged as a prevalent developmental challenge among college students. To clarify its underlying mechanisms, the present study investigated the relationship between cumulative ecological risk (CER) and PMSMU among college students, focusing on the mediating role of fear of missing out (FoMO) and the moderating role of core self-evaluations (CSE). A survey was conducted among 701 college students from two universities in Guangdong Province, China. The results revealed that CER was significantly associated with PMSMU, and this relationship followed a “negative acceleration model”. FoMO partially mediated the relationship between CER and PMSMU. Furthermore, CSE moderated both the direct effect of CER on PMSMU and the effect of FoMO on PMSMU in the mediation model. Notably, for students with high CSE, the relationship between CER and PMSMU manifested as an inverted U-shaped curve: after reaching the saturation of CER, as more risks accumulated, the level of PMSMU decreased. These findings contribute to a deeper understanding of the underlying mechanisms of PMSMU and provide practical insights for developing prevention and intervention strategies for college students.</p>