The variation of the hydrological conditions of Dongting Lake and its influenceafter the operation of the Three Gorges Reservoir was studied in the present paper. Based on the actual measured hydrological and topographic data, the variations of Dongting Lake were analyzed, including the change of sediment at the three inlets, the relationship between water levels at Chenglingji and the shrinking of lake area, the uncertainty of water source of the three inlets river network. Under the regulation of the Three Gorges Reservoir and the cascade reservoirs in the upstream, clear water discharge and the smooth of runoff process result in flood accumulation in Chenglingji. The water resources of the three inlets has also been damaged. In the future, only Songzi estuary can maintain constant runoff to avoid river blanking. The deepwater region of the west, south and east Dongting Lake will transform into rivers. To alleviate the impact of these changes and adapt to the flood control near Chenglingji, improving and regional optimizing of the flood control engineering systems in the lake area should be implemented. Dredging in the downstream can improve water environment at the three inlets area. The water level in the outlet of Chenglingji can be adjusted so that to protect the water condition of Dongting Lake and to ensure the ecological security of the wet land.