Poor psychiatric status and sleep quality were common among frontline healthcare workers (FHWs) during the outbreak of the 2019 novel coronavirus disease (COVID-19), but the change in these mental health outcomes overtime remained unknown. This study compared the psychiatric status and sleep quality of FHWs during and after the COVID-19 outbreak in China. FHWs who volunteered to work in Hubei province (the COVID-19 epicenter) were assessed at baseline during the COVID-19 outbreak and re-assessed when they returned to their place of origin (Liaoning province) after the COVID-19 outbreak. Participants’ psychiatric status and sleep quality were measured with the Symptom CheckList-90 (SCL-90) and the Pittsburgh Sleep Quality Index (PSQI), respectively. A total of 494 FHWs was assessed at baseline and 462 at follow-up assessments. The prevalence of poor psychiatric status was 10.5% at baseline and increased to 14.9% at the follow-up assessment (P = 0.04). The corresponding figures of poor sleep quality at baseline and follow-up assessment were 16.4% and 27.9%, respectively (P < 0.001). Multiple logistic regression analysis found that severe fatigue (p = 0.003, OR = 1.266, 95% CI = 1.081–1.483), poor sleep quality (p < 0.001, OR = 1.283, 95% CI = 1.171–1.405), and history of pre-existing psychiatric disorders (p < 0.001, OR = 5.085, 95% CI = 2.144–12.06) were independently associated with higher odds of poor psychiatric status among the FHWs. Poor psychiatric status and sleep quality were common among FHWs during the COVID-19 outbreak, and the prevalence increased following their volunteer experiences. This suggests a critical need for longer-term psychological support for this subpopulation.