Based on data from the 2018 and 2020 China Longitudinal Aging Social Survey(CLASS), this study systematically investigates the impact of reemployment on older adults' participation in volunteer service, as well as the underlying mechanisms, using a Logit model and the Propensity Score Matching (PSM) method. The findings indicate that reemployment significantly promotes volunteer engagement of older adults, with high-frequency reemployment associated with increased frequency of volunteer participation. Mechanism analysis reveals that reemployment not only shapes positive psychological cues but also enhances the perception of social support, both of which promote elderly participation in volunteer service. Heterogeneity analysis shows that the positive impact of reemployment is more pronounced among older adults who reenter the workforce voluntarily, compared to those who do so passively. Further analysis indicates that reemployment among older adults has a stronger positive effect on participation in the informal type volunteer service than on participation in the formal type or dual-type volunteer service. To systematically activate the synergistic effects of reemployment and volunteer participation among older adults, it is recommended to remove participation barriers, reshape the societal recognition of “productive and joyful ageing”, and provide targeted incentive measures.