Reasonable demonstration of impact from dam- break flood is of great significance to dam risk assessment and emergency management. When it comes to cascade dams failure, a 1D and 2D hydraulic coupling model is established to analyze the propagation of dam failure flood and the inundation of floodplain downstream the dam. Taking cascade hydropower stations on W river as an example, the impact of dam failure under two reservoir operation conditions in flood season and non-flood season, is considered in the model. The propagation of dam break flood is analyzed from aspects such as the peak discharge, maximum water level and timing of flood on typical section. On this basis, taking the maximum inundated depth of the flood plain as a representative index, dam failure flood risk level is divided and the flood risk map is drawn. The areas with high risk in case 1 and case 2 are 431.31 km² and 575.76 km² respectively, indicating the great impact of dam break flood on downstream areas. The simulation results have important reference value for developing emergency plans as well as disaster prevention and mitigation.