In order to formulate the ecological water replenishment plan of northern cut-off river more scientifically，this paper takes the Beijing section of the Yongding River as the research area. Based on the established channel hydraulics model， it simulates and analyzes the ecological water replenishment process of the study area in 2019 and 2020， and quantifies the amount of evaporation and leakage loss during the ecological replenishment process accounting for about 20%. The larger the replenishment flow of Guanting Reservoir， the lower the reduction rate of the peak flow of each downstream section. In order to reduce the flow loss and improve the utilization rate of the replenishment flow， it is suggested that the replenishment flow should be between 40~80 m³/s under the condition of relatively abundant water source， and the method of alternating small flow and large flow replenishment should be adopted at the same time. Under the condition of limited water source， the alternations of high-flow and low-flow water replenishment can be considered proper to realize the optimization of ecological water replenishment benefit in the basin.