Abstract: In recent years, ultra-low frequency oscillations with lower than 0.1 Hz have appeared in the islanded HVDC sending system successively, which seriously endangers the safety and stability of power grid. The research shows that the damping and stability of power system could be deteriorated due to the water hammer effect in the hydropower units and the unreasonable settings of governor control parameters. Moreover, the ultra-low frequency oscillation might be caused by the over-high proportion of hydropower units. Aiming at this problem, a linear state-space model of a multi-machine system with PID-type governor of hydropower units was firstly established in this paper to solve the ultra-low frequency oscillation mode. Then, the damping characteristics of governor control system were analyzed by using a damping torque method. Meanwhile, a novel design method of governor power system stabilizer（GPSS） by using phase compensation principle was also proposed, which can increase the damping of governor control system to suppress the ultra-low frequency oscillation. Finally, the characteristic analysis method was adopted in a 3-machine 9-bus system. According to the analysis of the ultra-low frequency oscillation characteristics based on the participation factor and the root trajectory, the governor control system has deeply participated in the ultra-low frequency oscillation, and the damping ratio of ultra-low frequency oscillation mode can be increased by setting the governor control parameters reasonably. And the effectiveness of GPSS designed for suppressing the ultra-low frequency oscillation is verified by simulation in a single-machine single-load system and a 4-machine 2-area system.