Objective: To investigate the social support of medical students during the outbreak of COVID-19 and its mediating effect on the correlation between epidemic exposure and depression with anxiety. Methods: A total of 5 593 medical students were enrolled, and Social Support Rating Scale(SSRS), Patient Health Questionnaire-9 and 7-tiem Generalize Anxiety Disorder Scale were used to measure. Results: The social support score of SSRS of male students was lower than that of female students (t=-4.36, P<0.05) and juniors and seniors scored lower than other grades(F=4.84, P<0.05). Epidemic exposure proportion was positively correlated with both depressive proportionand anxiety proportion(r=0.05, 0.06, P<0.01), while epidemic exposure and depressive proportion and anxiety proportion were negatively correlated with social support score(r=-0.05, -0.25, -0.19, P<0.01). After controlling for gender and grade, the mediating effect of social support in the correlation between epidemic exposure and depression accounted for 26.6% of the total effect, and the mediating effect of subjective support and supportive utilization was 32.3% and 25.7% respectively. After controlling for gender and grade, the mediating effect of social support in the correlation between epidemic exposure and depression accounted for 20.0% of the total effect, and the mediating effect of subjective support and supportive utilization was 15.9% and 12.2% respectively. Conclusion Social support has partially mediating effect in the correlation between epidemic exposure with depression and anxiety.