Abstract: Use of seasonal design flood can improve the utilization of flood resources while satisfying the reservoir safety requirement. Rainfall is the dominant factor in flood formation and hence its multi-stage calculation is a prerequisite and basis for calculating seasonal design flood. This paper presents a study on the distribution of staged extreme rainfalls in flood season based on fuzzy set analysis for the catchment of Dahuofang reservoir. We analyzed the flood season staged characteristics of the reservoir using a fuzzy set method, and constructed membership functions for pre-flood season, main flood season, and later flood season, by the theorem of the unity of opposites of variable fuzzy sets. Then, a staged extreme rainfall fitting model was developed, based on fuzzy set analysis, and the distribution patterns of extreme rainfalls in each stage of the reservoir is discussed. The results show that Gumbel, P-III, Weibull and GEV all are applicable to the distribution of extreme rainfalls over the study area, but the GEV function is optimal. The design rainfalls calculated by this fuzzy staging method is higher for main flood season than those by the actual staging but lower for later flood season, giving estimates favorable to the flood control and beneficial use of reservoirs.