Based on the basic theory and method of water resources system sustainability evaluation, we analyzed the water resources system sustainability of Beijing in 2025 under different water supply conditions and water demand schemes, and studied the impact of South-to-North Water Diversion(SNWD)on the future water resources system sustainability of Beijing, as well as determined the threshold value of water transfer scale of SNWD under different development scenarios from the perspective of sustainability. The results showed that SNWD can improve the future water resources system sustainability of Beijing in general. However, the sustainability of water resources system in Beijing is not optimistic under high water demand schemes or when the Danjiangkou Reservoir encounters an extremely dry year. In the future, we should reasonably increase the water transfer scale of SNWD and strengthen the emergency safeguard measures and strategic reserve of water resources on the basis of scientifically formulating water demand schemes, so as to improve the sustainability of water resources system and realize the sustainable development of water resources in Beijing.