The operation of Three Gorges Project (TGP) changes the flow and sediment conditions in the downstream river channel of the dam, and will further affect the hydrologic regime of its downstream river channel. Using one  dimensional mathematical models of flow and sediment, the changes of flow, water level and sediment load influenced by TGP operation along the downstream main river during high, average and low flow years are respectively calculated and analyzed. Results reveal that the flow increases during low-water and water  discharging seasons, and the downstream streamwise low water level of the dam rises accordingly. During storage period the discharge flow decreases and streamwise water level drops. During a low-flow year, the impoundment of the reservoir causes an excessively small inflow into the downstream river channel of the dam (with the average discharge at Datong hydrometric station less than 9 000m³/s in late October). Whereas during the storage period of an average flow year, the water level at middle and lower Yangtze River drops substantially with the streamwise water level drop in the range of 1.71 -3.68m , which influences the navigation and water supply in the downstream river channel of the TGP dam.