The Three Gorges Reservoir (TGR) is endowed with many tributaries and large local inflow. The correct calculation of local inflow process is an important issue in the hydrodynamics calculation for the TGR as the amount and process of inflow directly concerns the calculation precision of reservoir regulation and storage. In this article, the daily inflow process in the TGR area is back-calculated on the premise of water balance by a 1-D mathematical unsteady flow model proposed by the author previously. The local inflow process is added to the calculated reach by being distributed to each tributary in correspondence to the proportions of catchment area. The effect of adding local inflow to each reach is analyzed. Results demonstrate that the calculation results of the discharge process of the Three Gorges Reservoir are in good agreement with the measured results after the proposed method is adopted.