How do illegitimate tasks in volunteering affect volunteer participation? Previous research has focused only on the unidimensional effects of illegitimate tasks on volunteer participation. This study used the Job Demands-Resources model to investigate the multidimensional effects of illegitimate tasks on volunteer participation and the potential mechanisms of the effects. Based on three waves of survey data from 1768 Chinese volunteers, we found that illegitimate tasks negatively affect volunteer attitudes and volunteer outcomes by reducing volunteers' psychological capital. This study develops a mediated model about the effects of illegitimate tasks on volunteer attitudes and outcomes. This study also makes related recommendations, such as asking volunteer organizations to offer stress-coping courses and encouraging volunteers to share their personal volunteering experiences.