Abstract:  We studied the variations of water quality and its related driving factors of Dongting Lake from 2015 to 2020. Four water quality indicators were selected, including TN, TP, CODMn and NH₃-N, to analyze the lake water quality variations before and after the operation of the Three-year Action Plan. Water pollution index w and composite pollution index γ were calculated to assess the water quality pollution level of the lake. The driving mechanism of water quality variations was studied combining with four hydrological indicators, namely water level, inlet flow, inlet sediment volume and sedimentation, and four anthropogenic indicators of sewage treatment rate, livestock excretion, fish production and fertilizer application. The results showed that: (1) During the study period, the water quality of Dongting Lake showed an improving trend (p<0.05). The TN, TP and NH₃-N concentrations showed a significant decreasing trend, which were significantly reduced after the Action Plan. The composite pollution indices of Dongting East, Dongting South and Dongting West were respectively 0.86, 0.78, 0.78, and the pollution level of Dongting Lake was much higher in the eastern than in the western lake basin. (2) The water level, inlet flow, and inlet sediment volume were relatively stable, while sediment deposition rises significantly from -1 528.2×10⁴ t/a to 1 211.7×10⁴ t/a. (3) Livestock excretion, fertilizer application, sewage treatment rate, water level, and sediment deposition were the vital driving factors influencing the change of water quality. It was recommended that subsequent management be consistently strengthened to control the exogenous nutrients. Meanwhile, the hydrological conditions of Dongting Lake should be improved to enhance its hydrological connectivity.