Abstract: There are two problems in the calculation of water flow evolution between cascade hydropower stations in the basin. One is that due to the imprecision of storage capacity curve,discharge characteristic curve and unit consumption characteristic curve,there are inevitable errors in the calculated outgoing water volume and the change of storage capacity during the period,resulting in water imbalance between cascade hydropower stations. Second,the“sawtooth”phenomenon in the process of inflow flow obtained by short-term calculations is serious.To solve the two problems,this paper proposes a daily inflow forecasting method affected by the regulation of the upstream reservoir. The inflow is divided into two parts:the inflow of the upstream reservoir and the runoff of the interval. Taking the day as the length of the period,and based on the water balance theory,Muskingum confluence coefficient methodand API model,a correction coefficient is introduced into the river routing of the outflow of the upstream reservoir to deal with the problem of interval inflow or diversion,API model is used to calculate the interval runoff process. The practical application of Xijin Reservoir in Yujiang River Basin shows that the periodic Nash-Sutcliffe efficiency coefficient（NSE）is 0.943 and the inspection period Nash-Sutcliffe efficiency coefficient（NSE）is 0.914. This method fully considers the basic principle of hydrology and the actual operation of hydropower stations and reservoirs in the basin,which can effectively solve the problem of water imbalance between cascade hydropower stations and effectively avoid the fluctuation of inflow in a short period of time.