In order to achieve both hydropower generation and water supply，some hydropower stations with high water quality provide water to downstream areas by drawing water from the headrace tunnel． This paper takes the South Water Ｒeservoir Project as an example to study the transient process of the water conveyance system combining power generation and water supply by using the numerical method． Firstly，a mathematical model of the hydropower and water-supply system of the selected project is established． Then，the adjusting method of the initial parameters of the steady flow case and the interaction of hydropower and water -supply systems in the transient process under the three operation modes are analyzed． Finally，the numerical analysis ideas for transient process are induced in order to provide a design basis for this type of power generation and water supply engineering