Based on the measured hydrological and topographic data of the Three Gorges Reservoir, the dynamic reservoir capacity in the mainstream was calculated by a one dimensional numerical model, and the dynamic reservoir capacity of each characteristic reaches was fitted after comprehensive analysis. The study shows that the dynamic reservoir capacity in the mainstream of the Three Gorges Reservoir can be expressed approximately with good accuracy through the water level relationships from representative hydrological stations. In the case of a constant water level in the front of the dam, the dynamic flood control capacity decreases with the increase of discharge, and the difference between dynamic and static flood control capacities increases with the increase of discharge.