Abstract: Rainstorm is an important factor leading to debris flows in mountainous areas, prediction on debris flow erosion range under different rainfall conditions is still the focus of debris flow prevention and control. In order to prevent and reduce disasters, it is necessary to design a reasonable dam scheme. Taking the debris flow of Daping gully in Guangzhou City as an example, the movement process and prevention of debris flow are numerically simulated by using the shallow water flow modeling technology with complex topographic conditions. The results are as follows: ① the flow depth and velocity of debris flow increase with the decrease of rainfall frequency under different rainfall thresholds. When the rainfall frequency is 0.5%, the length of debris flow accumulation along the river channel will be 800 m, which is of great danger. ② Danger levels of the gully are clarified based on flow depth according to the simulation results,and a debris flow hazard index (HI) is proposed. ③ Combined with the Terrain Reconstruction Technology, different retaining dams are simulated. In the simulation, retaining dams with different heights are set at points A and B in the gully, and the best scheme is to set a retaining dam of 15 m at point A. The research results can provide reference for similar cases at home and abroad.