In the hydro-turbine governor system model， the strong non-linearity of hydro turbine prime mover， including the hydro turbine and conduit system， has significant impact on the stability computation of the governor system model. The main characteristics of hydro prime mover are non-linearity and water hammer， especially in the region with the gate opening more than 50%， that cause the same increase in gate position to yield less power. A hydro turbine prime mover model of governor system for power system stability computation， i.e.， a nonlinear model named servo position VS power output curve plus ideal turbine model is established by deducing the theoretic model of hydro turbine prime mover and modified using the field test results. According to the similarity between the real turbine and the model turbine， by using the hill diagram and doing field test， the curve under the maximum head and minimum head condition can be obtained to meet the different needs of power system stability computation.