Abstract: Risk of the construction diversion system is controlled by the real capacity of flood discharge and flood inflow,which has uncertain and dynamic pnperties.The paper uses the Monte-Carlo method to simulate the flood during construction and discharge process of diversion structure,analyaes statistically water level change of cofferdam in upperstream,and obtains a distribution function of the water level change.Based on the computational results,the risk of the diversion system are determined.The case study shows that the simulation model and analytical method are reliable and reasonable.