Raising rural residents’ income is central to narrowing the urban-rural income gap and achieving common prosperity. Using county-level panel data from 2016 to 2023 and leveraging the 2021 implementation of the Rural Revitalization Key Support County Policy as a quasi-natural experiment, this paper employs a difference-in-differences (DID) framework combined with spatial econometric models to systematically evaluate the policy’s impact on rural residents’ disposable income, its regional heterogeneity, and spatial spillover effects. The findings are as follows: (1) The key support policy significantly increases per capita disposable income among rural residents in targeted counties, and this result remains robust across a range of sensitivity tests. (2) The policy enhances income primarily by promoting off-farm employment, expanding access to financial services, and increasing educational investment. (3) The policy’s effects vary significantly across regions, with stronger impacts observed in non-ethnic minority areas, non-revolutionary base regions, counties with low natural disaster risk, and those outside ecological function zones. (4) The policy generates significant spatial spillovers, positively influencing income growth in neighboring counties. These findings suggest that while maintaining overall policy stability, differentiated regional support mechanisms and cross-regional coordination should be strengthened to enhance the policy’s overall effectiveness.