Abstract: In this paper, the mathematical model of reheat steam turbine has been inferred by using the local linearization method. The model can consider the nonlinear of valve, pressure change in the front of valve and the coordination of the intercept valve. Using this mathematical model, and comparing the digital simulation result with result of field test, It proved that the model is rational reliable. Therefore, this provided a theoretical basic for the simulation research of the control system of reheat steam turbines.