Abstract: The safety of dam not only affects the benefit of the project, but also relates closely to the safety of people's life and property. Dam-break flood simulation can evaluate the impacts of dam-break, and it is of great significance to the making of emergency plans so as to control flood and reduce disasters. Researches are carried out on the dam-break flood developing process in the downstream of the Minzhi reservoir based on the MIKE FLOOD model, which couples MIKE 11 and MIKE 21. By adopting two methods, a sudden dam-break method (for sudden partial dam-break as well as sudden full dam-break) and a gradual dam-break method, simulations of the flow graphs at the breach dam under four operating situations and the flood developing process in the downstream are respectively made in this study. The research results show that the peak flow of the sudden dam-break is larger at the beginning of the dam break, and the gradual dam-break peak flow is relatively small when the deformation of seepage failure develops to the upper part of the dam. And then, as the water level in the reservoir falls gradually, the flow discharge becomes smaller, until the reservoir is emptied. The dam-break flood has a greater destructive power against the upstream area of the Hengling village, where the submerged depth is larger. The flood velocity is up to 5 m/s in the residential and commercial areas of the middle reach of the Minzhi River. The flood has a certain damage to the buildings. The left side of the Xiangnan village is low-lying, the flooding is the most serious, and it still has a waterlogging depth of 3 m after the flood subsidence.