The paper uses the opening of high-speed railways as a quasi-natural experiment and uses panel data from 1,120 counties across China from 2003 to 2020 to construct a multi-period double-difference model to evaluate the impact of high-speed railway opening on county-level carbon emissions and analyze its mechanism. The study finds that the opening of high-speed railways can lower county-level carbon emissions; the impact of high-speed railway opening on county-level carbon reduction is heterogeneous, specifically, the carbon reduction effect of high-speed rail on the counties under the provincial capital is more obvious, and the positive impact of high-speed rail on county-level carbon reduction will gradually decrease as the distance from the center city increases. Compared with non-top 100 counties and counties with smaller population, the carbon reduction effect of high-speed rail on top 100 counties and large population counties is more obvious; high-speed rail mainly reduces county-level carbon emissions through improving economic efficiency, driving industrial structure upgrading, and enhancing market potential. The construction of high-speed railways has important implications for promoting the high-quality development of county-level areas. Therefore, it is necessary to further improve the railway network and other infrastructure construction, promote the extension of high-speed rail to more county-level areas, and strengthen exchanges and cooperation between county-level areas and the center city.