Background: Although professional identity is a strong predictor of career choice, only a few studies report on career attitudes of medical students during a public health emergency. This study investigates the changes in medical students’ professional identity and career attitude during the COVID-19 pandemic, evaluates their mental health and social support system under stress, and explores the relationship between their career attitude and other factors. Method: An online survey of 6226 Chinese medical students was conducted to collect information on demographics, professional identity, and career attitudes. The collected data were assessed using the Patient Health Questionnaire, the Generalized Anxiety Disorder Scale, and the Social Support Rating Scale. Results: The results revealed that most participants had their professional identity strengthened, did not change their career attitude, and preferred to participate in the frontline during the COVID-19 pandemic. Low social support, depressive symptoms, male gender, and senior year residency were factors that affected career attitude negatively. Conclusions: After the outbreak of the pandemic, it was necessary to carry out diversified professional identity research to support medical students, especially those with low social support and depressive symptoms.