Abstract: Sanjiangyuan National Park is a vital ecological security barrier in China, and managing it sustainably is crucial for maintaining ecological security and promoting regional development. However, the ecological and productive functions of the grasslands within the park are not currently well understood. A macro-scale analysis of ecosystem functions is urgently needed to optimise the spatial zoning of grassland resources. This study is based on the theoretical framework of multifunctional target management for grasslands. Using an ecosystem process model, the study calculates the ecological functions (e.g. soil conservation, wind erosion prevention, water retention, and wildlife maintenance) and productive functions (e.g. forage production and stability) of grasslands in the Sanjiangyuan region. It then constructs a spatial zoning framework for optimally protecting and utilizing grassland resources and proposes pathways for conserving biodiversity in the Sanjiangyuan National Park. The results indicate that the Sanjiangyuan region′s theoretical carrying capacity based on remote sensing and climate data is 23.99 and 34.39 million standard sheep units, respectively. However, the measured actual carrying capacity is only 19.7 million standard sheep units, suggesting that, while the grasslands generally fall within a reasonable carrying capacity range, there are imbalances in their spatial distribution. The spatial distribution pattern of both the ecological and productive functions of grasslands is higher in the southeast and lower in the northwest. A trade-off analysis of ecological and productive functions suggests that zoning the Sanjiangyuan grasslands into three categories—protected, natural grazing and cultivated—and implementing zoned management would benefit regional ecological security and the sustainable development of livestock farming. Protected grasslands are primarily found in the western and northern regions, accounting for around 56% of the total grassland area. Meanwhile, natural grazing grasslands are mainly located in the eastern and southern regions, accounting for around 36%. The remaining grassland areas (making up around 8% of the total grassland area and primarily found in the Nangqian, Gande, Banma and Henan regions) could be developed into cultivated grasslands, balancing production utilisation and ecological protection. This study quantitatively assessed the spatial differentiation characteristics and trade-off relationships between the ecological and productive functions of grasslands. Based on the 'three-grass coupling—three-life synergy' development concepts framework, it proposes an optimised resource protection and utilizaiton scheme for grasslands. This provides a specific scientific basis for decision-making and practical references for the conservation of grassland biodiversity and the management of ecosystems in the Sanjiangyuan National Park.