Narrowing regional development gaps is a critical pathway to achieving common prosperity. Since 1996, China has formally implemented an interprovincial paired assistance policy, under which developed eastern provinces provide systematic support to less developed central and western provinces in terms of capital, industry, and talent, forming a horizontal collaborative governance mechanism with Chinese characteristics. However, the actual effects of this policy on promoting income growth and narrowing regional income gaps remain inadequately assessed empirically. In particular, whether the policy simultaneously achieves its wealth-enhancing and common-prosperity functions, and through which mechanisms these effects materialize, require further investigation.Using interprovincial panel data from 1992 to 2023 in China, this paper constructs treatment variables based on the dynamic adjustments of pairing relationships over time and employs a staggered difference-in-differences (DID) model to systematically evaluate the wealth-enhancing and common-prosperity effects of the paired assistance policy. The findings are twofold. First, the paired assistance policy significantly increases both urban and rural per capita disposable income in recipient regions, with a larger marginal effect on rural residents, demonstrating a wealth-enhancing effect. At the same time, the policy significantly narrows the rural income gap between recipient and donor regions, while its effect on the urban income gap is not significant, demonstrating a common-prosperity effect. Second, the policy generates assistance effects in recipient regions through three pathways, namely improving infrastructure, promoting capital agglomeration, and facilitating technological innovation. Furthermore, it achieves the common-prosperity effect on rural residents by narrowing the technological innovation gap between recipient and donor regions.Compared with existing literature, this paper makes three marginal contributions. First, from the dual perspective of wealth-enhancing and common-prosperity effects, it simultaneously examines the absolute income-increasing effect and the relative gap-narrowing effect of the paired assistance policy, addressing the limitation of previous studies that focus on a single dimension. Second, using long-span interprovincial panel data from the inception of the policy in 1996 to 2023, it fully accounts for the dynamic adjustments of pairing relationships, thereby enhancing the external validity of the findings. Third, it systematically identifies the differentiated mechanisms through which the policy generates wealth-enhancing and common-prosperity effects, providing a clear theoretical basis for policy optimization.From the dual perspective of wealth-enhancing and common-prosperity effects, this paper conducts empirical analysis using long-term dynamic pairing data, offering new empirical evidence and a comparable analytical foundation for theories of regional coordinated development. The findings also provide insights for improving the interprovincial horizontal collaboration mechanism: future assistance programs should more precisely match supply and demand structures so that all urban and rural groups can share in the policy dividends. At the same time, assistance policies should shift from resource injection to capacity building, placing industrial cooperation and technology transfer in a more prominent position. Performance evaluation should move from input-oriented to outcome-oriented assessment, focusing on poverty reduction and the narrowing of regional disparities.