Affected by the terrain, reservoirs are taken as the main water sources in the Yunnan mountainous area. Because the most reservoirs were built many years ago, there is a greater risk of dam break under the influence of under the influence of precipitation concentrated in the monsoon climate. Therefore, it is important to make dam-break flood routing. In this paper, a dam-break flood routing model was used for the actual conditions of the Yuannan mountainous area and the Hexi Reservoir in Changning County was taken as a study case to determine the relevant parameters. The results show that if dam break occurs, reservoir will collapse fully, with a great amount of flood flow and rapid transfer.