Abstract: Study on the flood routing caused by dam (cofferdam) failure plays an important role in the early warning and flood control decision for the downstream area. Based on HEC-RAS, an one-dimensional unsteady hydrodynamic model, the flood of cofferdam-break during the construction period of a hydropower project was simulated by considering different reservoir inflow and cofferdam failure modes. The flood peak discharge and flood peak appearance time at the cofferdam cross section and downstream area under different cases were analyzed. The simulation results can provide scientific foundations for the pre-planning of flood safety and emergency treatment and reference for similar projects.