As the Three Gorges Reservoir is a channel reservoir, the static reservoir cubage method is not suitable for the accurate calculation of the reservoir water level. In this paper, a hydrodynamic model is built based on one dimensional unsteady flow. The model can predict the processes of water level and flow in front of the reservoir by simulating the process of flood routing in the river channel. Moreover, this model is validated by computation result using the impoundment data from 25 September to 3 October in 2007 and the comparison with observed data. It is manifested that this model offers good accuracy and can be applied to reservoir operation.