The Three Gorges Reservoir area has numerous tributaries and a large inflow rate between sections. Accurate calculation of the inflow process between sections is an important issue that needs to be considered in the water and sediment calculation of the Three Gorges Reservoir. This article uses a one-dimensional unsteady flow mathematical model of the main and tributary rivers of the Three Gorges Reservoir to obtain the interval flow process of the Three Gorges Reservoir through back calculation, and compares and analyzes the influence of interval flow on the accuracy of the Three Gorges Reservoir sedimentation simulation calculation. The results show that after considering the interval flow calculation, the calculation results of the outflow flow process of the Three Gorges Reservoir are in good agreement with the measured results, and the simulation accuracy of sedimentation in the Three Gorges Reservoir area has been improved.