With the operation of Three Gorges Project(TGP), water regime of rivers and lakes downstream of the dam changed with the reservoir regulation. The lowering of water level becomes normal after flood season compared with the years before TGP. Here, the water storage of TGP in 2006 was selected as a case to quantitatively investigate the effects of TGP on water regime of Lake Dongting. We applied the coupled hydrodynamic analysis model for the middle Yangtze River to compute the variation of water regime induced by the water storage. During impoundment, the reduction of the water level and discharge are 1.32 m and 466 m³/s, respectively, at Chenglingji, the outlet of Lake Dongting. The bottomlands inside the Lake Dongting are exposed ahead of time and the lake area with over 5 inundation days reach to 287 km². Results also indicate that:1) the impact of TGP on water regime varied with locations:east Lake Dongting takes the first place, the east of South Lake Dongting and the north of West Lake Dongting second, and the west of South Lake Dongting and the south of West Lake Dongting third. 2) The bottomlands which strongly affected by the impoundment are mainly located at East Lake Dongting, South Lake Dongting and the both sides of flood ways. 3) Two influencing mechanisms on water regime are identified:one is the increase of the lake discharge induced by the rapid drawdown of the mainstream of the Yangtze River; the other is the reduction of inflow from the Yangtze River via Sankou distributary channels.