Although most people are unwilling to work overtime, the phenomenon of voluntary overtime has aroused the attention of scholars. Studies have shown that voluntary overtime can reduce the increase in job burnout caused by overtime work, but little empirical research was made to elaborate the mechanism through which the motivation of overtime affects employee’s burnout. Besides, the presence or absence of rewards for overtime was associated differently with burnout. Therefore, the present study linked the self-determination motivation of overtime to emotional exhaustion on the basis of self-determination theory, and approached perceived task accomplishment as a mediator between them. Then, this study examined the interactive effects of self-determination motivation, hours and rewards of overtime on emotional exhaustion. Data for this study were collected through a two-wave survey among 508 employees working in different organizations and industries. At Time 1, employees were asked to report demographic information, self-determination motivation of overtime, overtime hours and the level of rewards for overtime. After 4 weeks (Time 2), participants were asked to evaluate their perceived task accomplishment and emotional exhaustion. The final sample consisted of 369 overtime employees (51.8% were female, mean age = 33.76 years), who worked for 8.03 extra hours per week on average. With hierarchical regression and bootstrapping method, the results of data analysis show that: (1) Self-determined motivation of overtime has a negative effect on emotional exhaustion, and perceived task accomplishment plays a partial mediating role in this process. (2) Overtime hours and rewards for overtime have positive and negative effects on emotional exhaustion respectively. (3) There are interactive effects among self-determination motivation of overtime, overtime hours and rewards for overtime on emotional exhaustion. Specifically, Long overtime hours strengthen the negative impact of self-determination motivation of overtime on emotional exhaustion, while high self-determined motivation and high overtime rewards weaken the positive impact of overtime hours on emotional exhaustion. Meanwhile, rewards for overtime has indirect negative influence on emotional exhaustion through self-determination motivation of overtime. The findings of this study contribute to overtime literature. First, the motivation of voluntary overtime behavior is further explored. By calculating the self-determination index (SDI), the motivation of overtime work is transformed into a continuous variable that can be measured, which helps to identify employees’ willingness to work overtime more accurately. Second, through exploring the mediating role of perceived task accomplishment, this study uncover the underlying mechanism between self-determination motivation of overtime and burnout. Third, the research found that the three factors of overtime — motivation, hours and rewards have a joint influence on employees’ burnout. This study also gives practical advices to organizations and their managers. Firstly, in order to improve employees’ occupational mental health, unnecessary overtime hours should be minimized through the improvement of work efficiency by applying new technologies and optimizing management processes. Secondly, enterprises are encouraged to take measures to improve employees’ work motivation and proactivity, such as implementing employee stock ownership plan, which could share the success and benefits with employees and offer high rewards for overtime that is sufficient to satisfy employees. The forms of overtime rewards are not limited to economic benefit, but also include job promotions and personal honors. Lastly, the task assignment should consider task urgency and workload simultaneously.