In response to the complex process of urban surface drainage and the difficulty in obtaining comprehensive data on drainage networks, a refined model of the whole process of rainstorm and flood in the Shenzhen Shawan River Basin of Shenzhen City was built based on three simulation methods, including physical mechanism-based drainage “surface water drainage network river channel” using inlets/inspection wells/drainage outfalls, simplified drainage “surface water → drainage network” based on catchment areas of inlets/inspection wells, simplified drainage “surface water → river channel” based on catchment areas of drainage outfalls. The rationality of the model was analyzed and verified by using the design data and the “9·7” extremely heavy rainstorm data in 2023. The results show that the absolute error of the water level simulation result at Danzhu Hydrological Station is 0.37 m, and the relative error is 6%, indicating that the model has good applicability. As the rainfall return period increases, the submerged area and depth of the Shawan River Basin continue to increase, and the accumulated water is mainly distributed in the northern, eastern, and southeastern parts of the basin. The number of full pipes in the pipeline section and the number of overflow nodes show an increasing trend and tend to be concentrated in distribution. When the rainfall return period is less than 20 a, the flood pressure in the river channel is relatively low, and the highest water level is significantly lower than the elevation of the embankment top. When the rainfall return period is greater than 50 a, the water level in the river channel is higher, and the flood pressure in the river channel is greater.