Abstract: Expanding rural consumption plays a critical role in building a new development paradigm; however, the lagging rural transportation infrastructure constrains the consumer market of agricultural counties. This study took the national demonstration county creation policy for “Four Good Rural Roads” as a quasi-natural experiment, and based on China’s county-level panel data from 2013 to 2023, employed the PSM-DID method to systematically examine the impact of rural transportation infrastructure improvement on consumption expansion in agricultural counties, and further analyzed its causal effects, mechanisms, and spatial spillovers. The findings were as follows: the creation of “Four Good Rural Roads” demonstration counties raised the total retail sales of consumer goods in agricultural counties by approximately 9.75% on average, and increased the per capita consumption level by approximately 9.6%; mechanism analysis indicated that the policy effectively promoted county-level consumption expansion by increasing the disposable income of urban residents; heterogeneity analysis showed that the counties in western regions and those with a high degree of agricultural dependence experienced more significant consumption expansion effects; the creation of demonstration counties had a significant positive spillover effect on consumption growth in neighboring non-demonstration counties. This study provides empirical evidence for optimizing the investment layout of rural transportation infrastructure and unlocking county-level consumption potential, and offers certain policy implications for improving the mechanism for expanding consumption effects.