Abstract: The middle route of the South-to-North Water Diversion project (SNWDP-MR) significantly influences the water use patterns and hydrological cycle structure of the Beijing-Tianjin-Hebei(BTH) region. To quantify the effects of this route on the regional water balance, in this study, a coupled surface–groundwater hydrological model (MODCYCLE) suitable for complex canal network systems was developed, and simulations were conducted to analyse the temporal evolution of hydrological fluxes and water storage during two periods: 2005—2014 (prediversion) and 2015—2019 (postdiversion). The results show that the SNWDP-MR effectively alleviated the regional water deficit, reducing the average annual water loss in the BTH region by approximately 2.70 billion m³, including a reduction of 1.86 billion m³ in shallow groundwater loss, demonstrating a significant recovery of shallow groundwater. In addition, the surface water and deep groundwater conditions also improved. However, the region remains in a long-term state of water scarcity. Overall, while the project plays a key role in both current and future water resource allocation, coordinated measures such as water conservation and controlled groundwater extraction are essential to ensure the sustainable restoration of the groundwater system.