Abstract: Under the background of tightening resource and environmental constraints and the strategic upgrading of grain security, scientifically assessing grain production efficiency of Sichuan’s “Tianfu Granary” and exploring pathways for its improvement are of great practical significance for achieving agricultural green transformation and sustainable development. Based on a survey data from 1,041 rice-growing households in Sichuan Province, this study employs an SBM-Undesirable model to measure and decompose grain production efficiency under six types of resource and environmental constraints and to identify its influencing factors. Results indicate that: 1) Under resource and environmental constraints, grain production efficiency in the “Tianfu Granary” is generally low, and failure to simultaneously incorporate multiple constraints leads to an underestimation of efficiency; 2) Grain production efficiency exhibits pronounced regional heterogeneity, following the pattern of Panxi Economic Zone > Chengdu Plain Economic Zone > Northeast Sichuan Economic Zone > Southern Sichuan Economic Zone; 3) Efficiency decomposition reveals that inefficiency in grain production mainly originates from input redundancy, with land input redundancy being the most prominent; and 4) Meteorological disasters and labor outmigration constitute the primary obstacles to improving grain production efficiency under resource and environmental constraints. Based on these findings, this study recommends improving mechanisms for the identification and monitoring of environmental constraints, enhancing green agricultural production capacity, optimizing the allocation and organization of production factors, establishing dual safeguard systems for labor availability and disaster prevention, and promoting regionally differentiated governance. These measures will help facilitate the coordinated transformation of factor structure and green production practices, thereby accelerating the development of a higher-level “Tianfu Granary”.