Abstract: The broken of "11.3" Baige barrier lake on Jinsha River resulted in the ultra-normal flood in hundreds of kilometers of the downstream. Accurate simulation of the flood is the basis of the flood forecast and the development of the response measures. The flood routing law in the downstream of the dam was simulated by MIKE11 and a one-dimensional dam break flood model developed by the author. The two models both solve the Saint-Venant equations, while the MIKE11 uses the Abbott six-point implicit difference method and the DBFM model uses a finite volume of the HLL+MUSCL interface interpolation method. The simulation results showed that the two models show differences in the flood propagation and the peak attenuation comparing with the measured data. Then, the influence of the factors such as the channel base flow, the momentum equation inertia, the channel roughness and the channel terrain on the simulation error of the flood were further analyzed, and the feasibility of using rolling forecast method to improve the flood forecast accuracy was demonstrated. In the end, the difficulties and requirements of the dam break flood simulation of the barrier lake in the mountainous area were put forward.