Striking a balance between agricultural production and ecological conservation has become a core imperative in global agro-environmental research, a challenge that is particularly pronounced in China, where the dual priorities of ensuring food security and alleviating environmental pressures must be urgently reconciled. The conventional land sparing/sharing (LSS) framework lacks quantitative assessment regarding the theoretical upper bounds imposed by regional resource endowments. We integrated the production possibility frontier (PPF) into the LSS framework and developed a cascading analytical framework of "boundary characterization-functional positioning-structural diagnosis". By fitting PPF curves, this study quantified the trade-offs between grain supply and comprehensive ecosystem services for agro-ecological composite units across China's ten Agricultural Resource and Environment Zones. On this basis, we identified the suitability of LSS strategies, measured synergy potential and trade-off intensity at the zonal level, and further determined unit-level efficiency types, dominant functions, and land use structure deviation. The results showed that: (1) the PPF curves across ten regions were predominantly convex to the origin, indicating an overall preference for land sparing strategy; (2) resource-use efficiency varied markedly among regions, with the Northwest Arid Region and the Inner Mongolia along the Great Wall Region exhibiting the highest trade-off intensity (0.786 and 0.549, respectively), whereas southern zones displayed relatively more balanced characteristics; and (3) structural diagnosis based on land use structure deviation helped to pinpoint areas where structural mismatched warrant priority attention. The proposed framework provides a scientific basis for reconciling agricultural production with ecological conservation and for implementing differentiated spatial governance.