<p>China’s express delivery business volume ranks first in the world. In an industry with a large business volume and few express delivery workers, the workload of these workers is gradually increasing. Drawing on the transactional process theory of stress coping, this study tries to explore the relationship between express delivery employees’ workload and job performance, as well as the mediating role of voice and the moderating role of proactive personality. Through a mixed-method study involving both a survey (targeting 688 express delivery employees and 128 direct supervisors; dyadic questionnaires were collected in two stages, one month apart, from Jingdong) and interviews (with an additional 48 express delivery employees and 20 supervisors from Jingdong), the results indicated that workload has a positive influence on employee voice, as well as on job performance. We also found that proactive personality moderates the relationship between workload and employee voice, suggesting that the positive relationship between workload and employee voice is stronger for employees with a high proactive personality. This study provides a valuable perspective on the workload of express delivery employees and expands the understanding of the effects of personality in both theory and practice.</p>