Abstract: The negative damping torque of the generator’s prime mover system is considered as the major cause of ultra-lowfrequency frequency oscillation（ULFFO）and tripping the primary frequency regulation of the units with negative damping can increase the damping of the system and suppress the oscillation.The key to emergency control is online evaluation of damping of prime mover systems.An online analysis and emergency control method of ULFFO based on transient energy flow is proposed.The transient energy flow method useful to the low frequency oscillation analysis is extended to the analysis of ULFFO.The consistency of the energy flow method and the damping torque method in analyzing the prime mover system’s damping in ULFFO is proved.An online evaluation method of the damping torque coefficient of the prime mover system is proposed according to the existing wide-area measurement system data,and the procedure of ULFFO emergency control is given.Simulation results are proof of the effectiveness of the proposed method which provides technical means for online control of ULFFO.