In today’s competitive workplace, employees are faced with more and more stressors and workload, which can lead to employee workplace procrastination. Procrastination refers to delaying the initiation or the completion of work assignment. Prior studies have shown that procrastination not only causes employees’ emotional problems but also decreases their well-being. Given the negative consequences of procrastination, it is of particular importance to explore its predictors and prevention methods. To data, some studies have revealed the antecedents of workplace procrastination, but they have mainly focused on the roles of individual and situational factors, resulting in the overlooking of the role of illegitimate tasks. Illegitimate tasks refer to those tasks that violate the norm about what can reasonably be expected from a given person. The purpose of this study is to explore the negative impact of this particular stressor (i.e., illegitimate tasks) on employees’ procrastination as well as its underlying mechanism and boundary condition. Drawing from conservation of resource, we develop a theoretical model explaining why and when illegitimate tasks lead to procrastination. Specifically, we expect that illegitimate tasks would be indirectly related to procrastination through ego depletion. Moreover, proactive personality might weaken the relationship among illegitimate tasks, ego depletion, and subsequent procrastination. To test our theoretical hypotheses, we recruited 196 employees from Chinese enterprises in different industries to complete questionnaires using a two-wave research design. Employees were invited to report their perceived illegitimate tasks questionnaire, ego depletion and proactive personality in the first data collection. Then, those participants who completed time 1 surveys were invited again to report their procrastination at work. We employed SPSS 24.0 and Mplus 7.0 to test our hypotheses. The results of regression analyses show that illegitimate tasks is positively related to procrastination. Ego depletion mediates the relationship between illegitimate tasks and procrastination. Proactive personality negatively moderates the direct relationship between illegitimate tasks and ego depletion, such that the relationship between illegitimate tasks and ego depletion is stronger for employees who have low proactive personality than who have high proactive personality. Proactive personality moderates the indirect relationship between illegitimate tasks and procrastination via ego depletion, such that the indirect relationship is stronger for employees who have low proactive personality than who have high proactive personality. Thus, our theoretical model was empirically supported. Our findings provide some theoretical and practical implications. First, by examining how the illegitimate tasks induce employees’ workplace procrastination, this study extends our knowledge about the antecedents of procrastination. Secondly, by identifying the mediating role of ego depletion in the relationship between illegitimate tasks and procrastination, this study opens the black box between illegitimate tasks and procrastination. Thirdly, this study reveals the boundary conditions of the above relationship (i.e., proactive personality), which helps alleviate the negative relationship between illegitimate tasks and procrastination. In addition, there are a number of practical strategies for organizational management practice. First, managers should reasonably assign tasks to their employees to avoid employees’ workplace procrastination. Second, it is necessary to create an environment that allows effective communication between managers and employees so as to reduce employees’ ego depletion. Finally, organizations should recruit employees with high proactive personality or train employees to be proactive.