[Objective] The Wudongde Reservoir is the uppermost cascade reservoir among the Wudongde, Baihetan, Xiluodu, and Xiangjiaba cascade reservoirs in the lower reaches of the Jinsha River. To support the operation scheduling of sediment discharge in the Wudongde Reservoir, a one-dimensional unsteady flow and sediment mathematical model for the mainstream and tributaries of the Wudongde Reservoir is established. The feasibility of the model is verified using measured data after impoundment, and then the sediment discharge characteristics of typical water and sediment processes during the initial impounding period of the Wudongde Reservoir are simulated. [Methods] The water and sediment processes in July and August of 2020, 2022, and 2023 were selected as typical water and sediment processes for model calculation. Among them, 2020 was a high-flow year, while 2022 and 2023 were low-flow years. [Results] 1) During July and August, the Wudongde Reservoir had strong sediment discharge capacity when the inflow volume was large. In July 2020, due to large inflow volume, the sediment discharge ratio corresponding to a water level of 945-975 m reached 32%-14%. During the impounding period in August, the impact of initial water level on sediment discharge capacity was not significant. In August 2020, due to large inflow volume, the sediment discharge ratio corresponding to an initial water level of 952-975 m was relatively large, ranging from 24.8% to 18.8%. During July and August, the Wudongde Reservoir could utilize high water for sediment discharge. 2) Due to significant reduction in sediment inflow, the siltation volume and proportion in the fluctuating backwater area of the Wudongde Reservoir during July and August were relatively small, providing favorable conditions for optimal operation of the reservoir. During the flood season in July, when the inflow volume was large and reservoir water level was >965 m, the siltation proportion in the fluctuating backwater area gradually increased but the siltation amount was small. When the inflow volume was small, the siltation pattern in the fluctuating backwater area was similar but the corresponding reservoir water level was 960 m. 3) At the beginning of August during the impounding period, when the initial water level was raised and the inflow volume was large, the impact on the siltation volume and proportion in the fluctuating backwater area was small. When inflow volume was small, the siltation proportion in the fluctuating backwater area increased correspondingly with the rise of the initial water level, but the siltation volume remained small. When the inflow sediment volume was relatively small, the inflow water volume was the main factor affecting the siltation proportion in the fluctuating backwater area in July. When the inflow sediment volume was relatively large, the inflow sediment volume was the main factor affecting the siltation proportion in the fluctuating backwater area in August. When the inflow sediment volume was relatively small, the inflow water volume was the main factor affecting the siltation proportion in the fluctuating backwater area in August. [Conclusion] With the significant reduction in inflow sediment volume after the impounding of the Wudongde Reservoir, even though the siltation proportion in the fluctuating backwater area increases, the increase in siltation volume in the fluctuating backwater area remains small, indicating room for further optimization of reservoir operation. Therefore, when studying the optimal operation of the Wudongde Reservoir, it is not sufficient to only consider relative indicators such as the sediment discharge ratio and the siltation proportion in the fluctuating backwater area. It is also necessary to consider absolute indicators such as the outflow sediment volume, the total siltation volume in the reservoir area, and the siltation volume in the fluctuating backwater area. This requires further long-term scouring and deposition calculations based on typical series years. Further in-depth research is needed on the optimization space for reservoir operation and specific regulation indicators under the permissible siltation principles of the Wudongde Reservoir.