Social workers’ mental health issues in China have contextually important meanings, which result from its situation of extensive development and limited resources, and result in more developmental problems in the next stage. To fill research gaps in perspectives, participants and empirical evidence, this study used the Conservation of Resources theory framework and a nationally representative sample to examine a moderated mediation model. A sample of 5,965 Chinese social workers was obtained from the China Social Work Longitudinal Study in 2019. The results indicated that the relation between person–job fit and social workers’ depression was partially mediated by their emotional exhaustion. In addition, the indirect effect was moderated by supervisor support, coworker support and manager support in the workplace. This study provides a resource-based mechanism to understand social workers’ mental health issues in the context of the developing world. Practical implications on improving social workers’ person–job fit, emotional exhaustion and work support were also discussed.