Eutrophication problem has always been one of the most difficult tasks in China''s water environment management. Hydrological frequency analysis was brought into eutrophication assessment in this paper. Monitoring indicators and evaluation criteria were normalized together with the empirical frequency. The assessment result is based on the combination of entropy weight and empirical frequency. According to eutrophication assessment in Chaohu Lake using this method, Chaohu Lake is classified in heavy eutrophication.