Abstract: Based on theory and general application method of fuzzy set analysis, fuzziness,randomness,processiveness of hydrology phenomenon and responding character of rainfall and runoff in flood seasons were considered in order to establish a serious of new accumulative rainfalls according to positive accumulative rainfall series and negative accumulative rainfall series.The series was used in stage division of flood season.This paper proposed the concept of highest thresholds of in and out flood season (ASMAX) to eliminate the impact of different thresholds.The concept and method were applied to clarify the stage of flood season in Yuqiao Reservoir watershed.It is identified that the flood season of this watershed should be between June 11 and September 18 and main flood period between July 21 and August 4.This conclusion is in accordance with statistic regulation of “July bottom and August top”,which means that the main flood season of Haihe watershed is between the last ten days of September and the first ten days of August.Accumulative rainfall series uncover processing character of flood season to certain extent, so it is suitable in stage division of flood season.〖

Key words: fuzzy set analysis, division of flood season, limit water level, highest threshold for the stage of the flood season〖