The invention relates to a prime mover speed control system additional damper control method. The mechanical power which affects the prime mover speed control system is calculated according to a pre-defined prime mover speed control system model. The mechanical power is decomposed into damping torque and synchronization torque components, so as to acquire compensated additional damping coefficients under power closed-loop adjustment and rotation speed closed-loop adjustment. According to the additional damping coefficients, the damping torque interval is determined. A speed control system additional damper model is built to carry out phase compensation on a compensation link corresponding to the phase frequency characteristic of the speed control system, so that the additional damping coefficients are positive. According to the invention, ultra-low frequency oscillation damping caused by the speed control system is improved.