Abstract: In order to make full use of water power and achieve comprehensive water resources regulation of river basin, cascade reservoirs were built in a large number of rivers in China. The risk analysis of dam-break flood of cascade reservoirs is of great significance. In the light of Songbaishan and Huaxi cascade reservoirs in Guiyang City, under the assumption of dam-break, the flood routing process was simulated with river channel dam-break model, and the water level variations of upstream Songbaishan reservoir, Huaxi reservoir area and downstream area were obtained. Based on the model calculation results, the impacts of different water level compositions of cascade reservoirs on the downstream were analyzed.