Abstract: Food security serves as the cornerstone of national security, and fiscal support for agriculture is one of the crucial means to safeguard it. Investigating the impact of fiscal support for agriculture on grain eco-efficiency is of great significance for ensuring national food security, promoting sustainable agricultural development, and optimizing existing fiscal policies aimed at agriculture. Drawing on panel data from 31 provinces (autonomous regions, municipalities directly under the central government) in China spanning 2012-2021, this study empirically analyzes the influence of fiscal support intensity and scale on grain eco-efficiency from the perspectives of both absolute and relative fiscal support levels. The findings reveal that: 1) The benchmark regression indicates a dual effect of fiscal support for agriculture on grain eco-efficiency, where the intensity of fiscal support significantly enhances grain eco-efficiency, whereas the scale of fiscal support exerts a significant inhibitory effect. 2) The heterogeneity test shows that the positive impact of fiscal support intensity on grain eco-efficiency exhibits regional heterogeneity, with the balanced areas (regions where production and consumption are roughly equal) experiencing the strongest effect, followed by major producing areas, and then major consuming areas. Conversely, the negative influence of fiscal support scale on grain eco-efficiency is observed across regions, with only the major producing areas showing a statistically significant effect. 3) The mechanism analysis highlights that agricultural technological innovation, farmland management scale, and farmer income are essential channels through which fiscal support for agriculture empowers grain eco-efficiency. To effectively leverage fiscal funds in enhancing grain eco-efficiency, it is imperative to intensify ecological subsidies, optimize fiscal expenditure structures, and strengthen the supervision and evaluation of fiscal support for agriculture. Differentiated fiscal policies tailored to various grain functional zones should be implemented. Furthermore, enhancing agricultural technological innovation capabilities, increasing farmers' income, and guiding moderate-scale farmland management can create favorable conditions for enhancing grain eco-efficiency.