Aiming at the problem of the river diversion and closure project of soil-rock cofferdam at a navigation and electricity junction in the Han River during flood season，the numerical simulation method is adopted to optimize the scheme，study the hydraulic characteristics of the berm advancing process and the impact of the closure on the bank slope，and reasonably determine the particle size of underwater riprap. The results show that：1）The reliability of the numerical simulation method is verified by comparing the measured data. 2） The water retaining capacity of the first stage cofferdam is predicted successfully，and the prediction accuracy of the water level is as high as 0.5 m. 3） The bidirectional vertical blocking method is adopted for the second stage upstream cofferdam，which reduces the scouring of bank slope by the closure. At the same time，the predicted flow velocity and the particle size of riprap can meet the construction requirements of the site，which provides a scientific basis for the construction organization design. The above conclusions have been successfully applied to the actual construction of the project and ensured a successful closure of the project during flood season.