The Dianzhong Water Diversion Project is the largest and most costly water resource allocation initiative in Southwest China. The Xianglushan Tunnel, a deep-buried structure located in Songgui Town, Heqing County, Dali Prefecture, is particularly threatened by high external water pressure. On-site monitoring indicates that in deep-buried tunnels with low rock mass permeability, the impact of construction drainage on water table is minimal, resulting in a low pressure head around the tunnel. A three-dimensional finite element numerical model was developed to study a typical tunnel section. The model revealed that pressure head distribution outside the tunnel is significantly influenced by tunnel drainage and the geological structure. In particular, under specific geological conditions, localized drainage areas may form at the top of the tunnel without affecting the water table. Increasing tunnel seepage during construction may indicate the presence of a water-conducting structure ahead of the tunnel, necessitating enhanced disaster prevention measures. This work provides a reference for the study of external water pressure in deep-buried tunnels.