As the only reservoir of overyear regulation in the Yellow River,Longyangxia Reservoir can take important roles in the unit water operation of the river.The operation of Longyangxia Reservoir,which considering water supply for the lower reaches means its water supply area is enlarged.In this case,what is the influence on the power generation of upper cascade power stations?How much is the water supply capacity added?All these are the focus questions considered by water and power departments.Based on"Yellow River Water Resources Simulation Model",this paper uses long series simulation method to calculate the benefits of the operation of Longyangxia Reservoir considering water supply for the lower reaches.The study indicates that Longyangxia Reservoir operation can consider the water supply for the lower reaches.In the dry years,it can reduce water shortage of the trunk steam as affe cted at the expensive of the fall of power.However the expensive is little.