In order to explore the impact of deep tunnel drainage projects on urban flood control and drainage situations, taking the South Drainage Deep Tunnel Project, the first deep tunnel drainage project under construction in the western part of Hangzhou, China, as an example, a hydrological and hydrodynamic model that incorporates the deep tunnel drainage project was constructed, and quantitative simulations were carried out under various schemes. The spatial patterns of the improvement in drainage capacity and drainage standards by the deep tunnel drainage project were analyzed, the essential differences in drainage effect between deep tunnel drainage projects and conventional drainage projects were revealed, and the general laws of the impact of deep tunnel drainage projects on flood control and drainage situations in mega cities are summarized. The results show that the deep tunnel drainage project can significantly improve the drainage standards in the western part of Hangzhou, with the drainage standards increasing from less than a 20 a return period to a comprehensive 20 a return period, and even reaching a 50 a return period in some areas. The improvement effect of the deep tunnel drainage project on the drainage capacity is related to the terrain and distance from the water inlet. The lower the terrain and the closer the distance to the water inlet, the more significant the improvement effect on drainage capacity. The difference between deep tunnel drainage projects and conventional drainage projects is that conventional drainage projects transfer flood risks from upstream to downstream, while deep tunnel drainage projects can eliminate local flood risks and maximize drainage benefits.