There are significant differences in the oscillation performance and the mechanism of the ultra-low-frequency frequency oscillations(ULFFO)from that of low frequency oscillations. The ULFFO problem is analyzed in the multi-machine system. In the ULFFO

all of generator speeds change coordinately. As a consequence

the system's frequency oscillates together. An equivalent method for ULFFO analysis is proposed to increase computation efficiency

in which the multi-machine system is equivalent to a single-machine system. Damping torques in the prime mover systems of the generator are decoupled. The sum of them and the damping torques in the electromagnetic power affect the damping ratio of the system. The results provide guidance for the analysis and control of ULFFO in the actual system.