There is significant interest among policymakers and academics about whether green agricultural subsidy policy (GASP) has achieved its intended effect of reducing agricultural carbon emissions (ACEs) in China. Based on the panel data of 172 prefecture-level cities from 2010 to 2021, this study evaluates the impact and mechanisms of GASP on ACEs’ reduction effect by employing the DID model. The results demonstrate that GASP does significantly reduce carbon emissions. Mechanism tests illustrate that GASP promotes carbon reduction via two mechanisms: enhancing agricultural technology progress efficiency and increasing scale management efficiency. Further heterogeneity analyses reveal that the carbon reduction effects of GASP are particularly prominent in the main grain-producing regions and in cities with higher levels of carbon emissions. The empirical tests and mechanism analyses in this paper can better clarify the function of GASP, enrich and deepen the existing literature, and provide some useful references for carbon reduction.