Objective: To examine the association of long working hours and shift work with occupational stress among medical staff in level A tertiary hospitals, so as to provide insights into promotion of physical and mental health among medical personnel. Methods: One level A tertiary hospital was sampled using a stratified cluster sampling method from southern and northern Xinjiang Uygur Autonomous Region, and all medical personnel were recruited from these two hospitals. Participants' demographics, working duration, and working in shifts were collected using questionnaires, and occupational stress was measured using the Core Scale for Measurement of Occupational Stress proposed by National Institute for Occupational Health and Poison Control, Chinese Center for Disease Control and Prevention. The associations of long working hours (weekly working duration of >40 hours) and shift work with occupational stress were examined using a multiple linear regression model. Results: A total of 2 529 questionnaires were allocated, and 2 262 were valid, with an effective rate of 89.44%. The respondents had a mean age of (35.12±8.71) years, and included 1 696 women (74.98%). Of all respondents, there were 722 doctors (31.92%), 1 033 nurses (45.67%), 361 medical or pharmaceutical technicians (15.96%), 1 808 with long working hours (79.93%) and 1 264 with shift work (55.88%). The score of occupational stress was (44.79±8.49) points, and the prevalence of occupational stress was 28.69% among respondents. Multiple linear regression analysis showed that after adjustment for age, marital status, length of service, position, smoking and physical exercise, long working hours (>40 h, β'=0.124; >48 h, β'=0.175; ≥55 h, β'=0.323) and shift work (β'=0.203) were influencing factors for occupational stress among medical personnel（P<0.05）; however, there was no interaction between long working hours and shift work (P>0.05). Conclusion Long working hours and shift work may increase the risk of occupational stress among medical personnel in level A tertiary hospitals.