To predict the risk assessment of a dam which constructs two special tunnels for unusual flood release, a method based on stochastic differential equation is developed in this paper. After analyzing the sensitivity of computational conditions on discharge risk, the requirement for useing two unusual tunnels in this project and their discharge risk are discussed, which provides a scientific basis for making decision on unusual tunnel design plan. The analyzed results show that the risk level of unusual tunnel plan can keep the same as that of the original design plan. Therefore, it proves that the unusual tunnel plan is reasonable.