Abstract: In order to accurately simulate the downstream flood routing of the dam-break flood after dam breach, the two-dimensional (2D) hydrodynamic model with HEC-RAS software is used to modify the breach progression curve of the rock-fill dam with face slab and design the working conditions of the overtopping of the Xiaojinggou concrete-faced slab dam with two gate openings. And the model is also used to simulate the downstream flood routing of the dam-break flood under the influences of reservoir flood discharge and generate the corresponding flood risk maps, the maximum velocity distribution maps and the flood detention time maps. The simulation results show that the submerged depth and scope of the dam-break flood in the plain and hilly areas of the middle and lower reaches vary greatly under different gate openings, and the maximum flow velocity and the flood detention time have little difference. From the simulation and analysis results it is found that the reservoir flood discharge can effectively reduce the threats of the dam-break flood to the people's lives and property in the downstream plain areas. The simulated results have important reference values for the evacuation and loss assessment of the dam-break flood.