For achieving the high-fidelity simulation of steam-turbine generator on the real-time simulator of coal fired plant, the paper establishs a group of state equations based on Parkg equation, which can accurately describe its transient process. Then a kind of time-saved numerical solution in terms of four-order Runge-Kutta method is proposed through the modified processing to the time-dependent system matrix A and control matrix B. Finally a program is coded to simulate the emergency load process of one 360MW steam-turbine generator, the result indicates that the method of this paper can exactly apply on the generator model of real-time simulator.