Abstract: Through fieldwork and water sample monitoring, the application of BP neural network model, evaluation method of lake eutrophication and ShannonWeaver index, the study separately evaluated the water environment quality, nutrient level and aquatic life status of Dongting lake before and after the impounding of The Three Gorges Project in typical years (1986,1991,1998,2004,2006 and 2010).The results indicate that after the impounding of the Three Gorges Reservoir, the water environmental quality get worse as a whole, the overlimit ratio of the water quality of all its sections has a significant increase and it presents mainly to N,P accompanied with organic pollution. The Nutrient index has risen.The comprehensive nutrition index of east Dongting Lake from 2008 to 2010 exceeded 50 and became Mild eutrophication. Cryptophyta has replaced bacillariophyta and becomes the dominant species of phytoplankton in Dongting Lake, where its biodiversity index has descended. The dominant species of zoobenthos is bellamya purificata in east Dongting Lake while branchiura sowerbyi in the west and south Dongting, whose species and density tend to fluctuate downward. The factors affecting the water quality of Dongting lake are not only the “three ports”, “four waters” pollutants, but also the pollution of the lake area and nonpoint source pollution, the crucial reasons are the great reduction of the amount of sediment in the lake and the change of land use pattern.