Taking a rockfill dam as a research object, the collapse modes of flood overtopping were considered. The flood process of rockfill dam break was calculated by DB-IWHR model under extreme conditions of the combination for flood limit water level, checked flood water level, checked flood discharge and multi-year average inflow. Flood evolution process was simulated using MIKE 21 to assess the downstream risk of flood inundation by taking the maximum discharge processes of historical floods of tributaries encountered with the flood peak of the main stream. The Centianhe Dam was selected as an example, in which different combinations of inflow floods and dam erosion features were determined according to the reservoir capacity and rockfill material parameters. The results show that the proposed dam break and flood routing models can effectively simulate the process of dam break and the characteristic of flood inundation risk in the downstream region. Finally, suggestions for safe operation of the dam are put forward and plans for flood avoidance and personnel transfer are correspondingly formulated.