In this paper，a onedimensional mathematical model is developed to study the sediment transportation law in the downstream reach of Sanshenggong Reservoir under different flow conditions and sediment concentrations. The sediment concentration，cumulative erosion and deposition，and erosion and deposition along the river section are counted，the erosion and deposition per length of the river is calculated，and the erosion and deposition characteristics of different river sections are quantitatively analyzed. The results show that under the same flow discharge，the larger the sediment concentration in entry，the larger the erosion and deposition along river and cumulative erosion and deposition. However，the sediment concentration after the 100 km downstream reach is prone to be consistent，having nothing to do with the entry sediment concentration. Meanwhile，the discharge flow and sediment content of the reservoir have a significant impact on the erosion and deposition of the downstream reach. When the sediment content of discharge is not more than 3kg/m3，the downstream river is dominated by erosion. Finally，the erosion and deposition change of the reach from the reservoir to the Sanhu Estuary are significantly affected by the upstream inflow and sediment conditions，and the balanced inflow and sediment coefficient of erosion and deposition is around 0.004kg·s/m6.