Under the background of deepening the construction of a unified national market, resolving the tension between establishing a unified, open, and orderly competitive market system and mobilizing local initiative is a theoretical and practical issue of high priority. This paper analyzes the pivotal role of the pairing assistance mechanism—characterized by horizontal fiscal transfers—in stimulating local initiative and advancing the development of a unified national market, from the perspective of intergovernmental fiscal relations. Theoretical analysis reveals that horizontal transfer payments embed rich fiscal and political incentives. They not only shape a “revenue-sharing” contractual structure between assisting and assisted governments but also embody the fundamental character of intergovernmental relations—central unified leadership with local hierarchical management. This mechanism simultaneously mobilizes local initiative and advances unified market development. Empirical analysis indicates that horizontal transfer payments significantly reduce market segmentation in goods and capital factors. However, this policy effect remains confined to paired cities and does not prompt recipient governments to relax market segmentation on a broader scale. Mechanism tests reveal that interprovincial trade flows and cross-regional corporate investment constitute the micro-level operational mechanisms through which horizontal transfer payments facilitate goods circulation and capital factor mobility, thereby advancing the construction of a unified national market.