Steam turbine valve discharge characteristics are among the important factors affecting the stability of the power systems. The influence mechanism of steam turbine valve discharge characteristics on the security and stability of the power grid is studied through the mathematical modeling of steam turbine and its speed control system， generator and excitation system. Mathematical analysis and simulation results show that cyclic fluctuations of generator active power might be caused by undesired steam turbine valve discharge characteristics. In view of this phenomenon， a modified control strategy of turbine governor modification system is proposed. This strategy can inhibit the excessive accommodation of proportion-integration-differentiation（PID） controller which is based on the power control mode， effectively increases the system damping， and suppresses power fluctuations of the prime mover.