On the basis of meteorology research results, the typical rainstorm areas of Zhejiang Province are presented by using the equivalent line technology in combination with the underlying surface condition. And then, taking the typical rainstorm area as an illustration, the rationality of flood season staging in Zhejiang Province is analysed by probability method. It provides a theory basis for renewing stage of flood season. This stage is benificial to making use of large-and medium-scale reservoir water resources. It is very necessary to relieve contradiction between supply and demand of water resources.