In order to overcome the problem existing in reservoir stage operation at present, a new method was proposed on the basis of probability combination principle in probability theory, the equivalent formulation of the reservoir flood control standard was given specifically, and the flood control water level of each stage was deter ? mined by the condition that the reservoir flood control standard is not reduced. Analyzed and calculated results obtained by? the example adopting 3-stage regulating flood indicate that the water level for flood control in the end of the flood season can be raised by? 1.80m most greatly, as a result, the multipurpose effectiveness of the reservoir will be increased greatly. The method can provide the reference for flood control optimal operation of the built reservoir.