Based on maximum entropy distribution of the highest water level in weir, in this paper we present a simulation method of construction diversion risk that intuitively describes the diversion risk rate function of operational cofferdam and accurately assess construction diversion risk. In our method, the highest water level distribution density function is derived by using the distribution of flood peak and flood volume with the maximum entropy principle. Meanwhile, combining the discharge uncertainty of diversion buildings, the method stochastically simulates the highest water level sequence in weir. Then the method is employed to calculate diversion risk rate. The analysis shows that the simulated construction flood can well reflect the measured flood series. Moreover, the derived distribution density function of the highest water level in weir is reasonable, which can fully reflect true situation of recent measured flood in weir compared with traditional methods. On the premise of accuracy, this method improves risk calculation efficiency and applicability. Therefore, the method provides support for risk analysis and program decisions of construction diversion.