Abstract: The BREACH model is widely used to simulate overtopping and piping dam-break model of the reservoir dams and obtain the relationships between breach flow and time. The MIKE21 model is commonly used to simulate the two-dimensional hydrodynamics of lakes, estuaries, bays and other areas and obtain the flood risk information elements within flood plains. Coupling analysis of two models has a positive effect in the protection of the reservoir dam safety operation and risk management. Based on the analysis of two models application examples in recent years, starting from the modeling principle of BREACH model, simulation of the breach dynamic flow data process and the breach evolution size with BREACH model can obtain the combining input parameters of BREACH model. Through ArcGIS preconditioning terrain file, the engineering structure parameters and hydrological data, a MIKE21 two-dimensional hydrodynamic model is developed, which is divided into the unstructured triangular grid and computational grid difference spaces, and via the Flow Model a two-dimensional hydrodynamic simulation file is generated for the dam-break flood. Coupling the two models by an iterative method to judge the breach flow differences (the differences between the flood flow of the grids where the breach is placed in MIKE21 model and the calculated breach flow in BREACH model) whether to meet the accuracy requirements. Finally, combining with the general engineering situation of the reservoir dams needed to be analyzed, the reservoir dam-break flood submerged water depth, submerged area and flood incoming time are calculated under the different risk conditions, and then the dam-break flood routing process is simulated. This method is successfully applied to the risk analysis and emergency plan preparation for Changzhuang reservoir in Zhengzhou City of Henan Province. The analysis results show that the method can objectively reflect the dam-break flood situations, being easy to use, and provide some technical supports in working out the emergency measures for the operation and management of the reservoir dam.