The partition of flood season of reservoir is favorable for mitigating the contradiction between preventing flood and infensifying water project and improving the flood resources utilize efficiency. The paper studied flood season aging of Shifosi Reservoir and Qinghe Reservoir by using the measured data of 45 years from 1956 to 2000 based on the method of fuzzy sets analysis ,and analyzed its rationality by the measured data of 45 years in accordance with computer program of method of fuzzy sets analysis method. The results show that the first ten-day of June to the second ten-day of July is pre-flood season, the third ten-day of July to that of August is main flood season, the first ten-day to the third of September is latter flood season for Shifosi Reservoir; As for Qinghe Reservoir, the first ten-day of June to the second ten-day of July is pre-flood season, the third ten-day of July to the second ten-day of August is main flood season, the third ten-day of August to that of September is latter flood season. Base on the reasonable test result of runoff data of 45 years,the staging result of flood season is feasible for Shifuosi and Qinghe reservoirs,which indicated that application of fuzzy sets analysis method in staging flood season in Liaohe River catchment is feasible.