Simulating the flood level at downstream cascaded hydropower station in mid-stage construction is of critical significance once the adjacent upstream cofferdam in simultaneous construction encounters standard-exceeding flood and overtopping failure which severely threatens the construction safety of the downstream plant. In line with the hydrodynamic theory, a mathematical model is built for simulating the process of water level variation in mid-stage construction period under the condition of cofferdam break of the upstream cofferdam. The model is applied to two adjacent cascade hydropower stations in Dadu River valley. Simulation results suggest that the model is feasible and effective. Compared with the calculation method based on natural flood flow, the proposed method is closer to the actual situation, and the obtained maximum water level in flood season is apparently larger, which is more conducive to the safety of the project. This study provides an important theoretical basis for the decision-making and flood emergency planning of downstream plant in mid-stage construction with cofferdam in the upstream.