Based on the water quality monitoring data of 14 monitoring sections of Dongting Lake in 1991-2015 and runoff and sediment discharge monitoring data of nine hydrological stations input and output of the lake, the temporal and spatial trends of water quality and characteristics of nutrient condition evolution in Dongting Lake were analyzed using single factor's evaluation and water trophic level index (TLI). The results showed that the overall water quality of Dongting Lake showed a significant deterioration. The water quality of Dongting Lake remained at a level of Ⅳ in the years 2011-2015, and the percentages of Ⅰ-Ⅲ level declined significantly from 1991 to 1994. The eutrophication of lakes gradually became mild eutrophication, and the main pollutants were TN and TP. From the spatial distribution, eastern Dongting Lake pollution was the most serious. Western Dongting Lake and Southern Dongting Lake were less polluted. Then the formation mechanism and the highest pollution factors of water quality change in Dongting Lake were reviewed. Industrial wastewater, domestic sewage and agricultural non-point source pollution comprised the main pollution sources. At the same time, water quality change was also adversely affected by the change of river-lake relation, especially the three outlets of the Jingjiang River. A series of prevention and control measures for water quality in the Lake District was later put forward, aiming at providing some reliable theoretical basis and technical support for water quality safety in Dongting Lake.