Low-frequency oscillation (LFO) of resonance mechanism could still happen after installing PSS which is the most common method of suppressing LFO. This paper deduces the mathematical expression of the amplitude of LFO of resonance mechanism and analyzes the influencing factors. Then, a design method of PSS for suppressing LFO of resonance mechanism is proposed. In this method, the object function is designed considering both the phase and amplitude frequency characteristic to provide better compensation angle and adjust the gain PSS provides. Besides, weight factors are obtained according to the resonance characteristics of each mode, so that the oscillation mode which causes more damage to the power grid can be better suppressed. Particle swarm optimization is used to search the optimal solution. The simulation results in four-machine and two-area system demonstrate that, compared with PSS designed according to negative damping mechanism, PSS designed by the proposed method has better damping effect on LFO of resonance mechanism without affecting the suppression effect of LFO of negative damping mechanism.