Quantitative and effective flood control operation of cascade reservoirs under emergency conditions can guarantee the safety of dams, reduce the social loss caused by flood disaster. A quantitative and practical risk analysis model of cascade dams overtopping breach (DAMSBREACH) has been developed based on the improvements to the analytical method of single dam breach. A single dam breach module, a flood routing module, a cascade dam overtopping and warning module have been assembled in the DAMSBREACH for predicting cascade dams overtopping flood and risk warning. The main improvements include a hyperbolic soil erosion model, the circular slip surface used in lateral enlargement modeling, and a numerical algorithm that uses velocity increment and allows straight forward calculation for the breach flood hydrograph. The DAMSBREACH is applied to Xiaerxia–Dawei–Shuangjiangkou of three cascade dams, which located in the upper reaches of Dadu river in southwest of China. The results show that only under the condition to open all release structures of Shuangjiangkou reservoir while Dawei dam starts to break, Shuangjiangkou reservoir will not be overtopped and the highest water level reaches 2507.56 m that is lower than the elevation of dam crest 2510.00 m. The integrated risk of cascade reservoirs has a direct relationship with the initial water level, uncertainty flood from the upstream, the discharge capacity and warning time of the reservoir. The risk analysis model of cascade dams overtopping break can help the decision-makers choose the best operation mode according to the characteristic of the reservoir under different emergency operating conditions.