There are two issues resolving for the varied operational strategies in different stages during flood season for a reservoir.One is to guarantee constant to the standard for flood control of the reservoir,another is to estimate stage design floods of the reservoir during flood season.This paper proposed a method to estimate stage design floods during flood season and an equivalent formulation of the standard for flood control according to the probabilistic combinatorial principle.Based on the data of a reservoir,the design floods and lower limit levels for flood control of the reservoir in different stages during flood season are estimated by the proposed method provided that the equivalent standard for flood control is equal to the standard for flood control.The results of the case study showed that the method and the equivalent formulation could provide a good fit to estimating stage design floods during flood season.