Abstract: Nianpanshan water complex is Class Ⅱ large (2) project,which takes cofferdam to block off the river bed,whose diversion design standard is P=10% for the whole year.When the cofferdam encounters excessive flood,the cofferdam can be broken to discharge flood according to the water situation.To predict the cofferdam break impact on downstream area,the paper establishes a dambreak flood routing model,which can carry upstream flood routing calculation,the breach development process simulation and downstream flood routing simultaneously.The calculation results show that the dam break flood wave will flatten in the process of propagation after the dam break due to the 10-year and the 20-year flood in the upstream,and will not exceed the safe discharge of Shayang reach.The section of Shayang River will exceed the safe discharge of 18 400 m³/s when the dam breaks due to the 50-year flood in the upstream,which is a big threat to the downstream area of Hanjiang River.The calculation results provide technical support for the beyond standard flood prevention of Nianpanshan cofferdam and the flood emergency work of Hanjiang River.