Based on organizational support theory and “stressor-strain-outcome” model, this study constructs a moderated mediation model to research the mechanism between workload boundary and employees’ emotional exhaustion. Using a questionnaire survey, the results show that there is a U-shape relationship between the workload boundary and the employees’ emotional exhaustion; and this relationship is mediated by employees’ role stress. Furthermore, boundary flexibility moderates the U-shape relationship between workload boundary and employees’ role stress. This means that the boundary flexibility can help reduce the negative effects of the workload boundary on the employees’ role stress, and compared with too strong and too weak workload boundary, the reduced negative effects are the most appropriate when the workload boundary is in the middle. Lastly, workload flexibility moderates the mediation effects of employees’ role stress.