To understand the changes of water quality in Lake Dongting before and after Three Gorges Project operation, the water quality in Lake Dongting was evaluated based on the water quality monitoring data of Lake Dongting from 1996 to 2013 by using Nemerow pollution index method, and the spatial and temporal variation characteristics of water quality and main pollutants in Lake Dongting were analyzed. The results showed that the I_P values of Lake Dongting ranged from 1.10 to 2.20, with an average of 1.63 from 1996 to 2013, the water pollution level was between light pollution and pollution, the water quality did not change significantly generally, but since 2010, the I_P values of Lake Dongting had been continuously under the average for many years, and water quality became better. Total phosphorus and total nitrogen were the major pollutants in the lake, the concentration of total phosphorus did not change significantly, but the concentration of total nitrogen showed a significant upward trend. Comparing with the period before Three Gorges Project operation, the concentration of total nitrogen in Lake Dongting and the I_P values and the concentration of total nitrogen in southern Lake Dongting increased significantly after Three Gorges Project operation, the water quality in southern Lake Dongting deteriorated significantly. The order of I_P values and the concentration of total phosphorus in different water phases changed from flood season> non-flood season to non-flood season> flood season, and in spatial distribution changed from western Lake Dongting> eastern Lake Dongting> southern Lake Dongting to western Lake Dongting> southern Lake Dongting> eastern Lake Dongting, respectively before and after the Three Gorges Project operation. Since 2010, the ranking order of I_P values had changed in spatial as eastern Lake Dongting> southern Lake Dongting> western Lake Dongting. The spatial and temporal changes of I_P value and the concentration of total phosphorus associated with changes of flow and sediment conditions in Lake Dongting before and after Three Gorges Project operation, the concentration of total nitrogen was less affected by the Three Gorges Project operation, mainly by nitrogen pollution of Xiangjiang River, Zishui River and so on in the four river basin.