Ultra-low frequency oscillation will cause overcurrent tripping of tie line and out of step splitting between system and unit or system, which will affect the safe and stable operation of power grid. Therefore, it is necessary to study the suppression measures of ultra-low frequency oscillation. Power system stabilizer can be used to suppress ultra-low frequency oscillation. Firstly, the mechanism of generator excitation system affecting ultra-low frequency oscillation is analyzed. When the load has voltage regulation effect, the generator excitation system affects the ultra-low frequency oscillation by affecting the load voltage and then the load power. Furthermore, the process of frequency deviation affecting electromagnetic power through power system stabilizer, excitation system, network and load is given. The damping coefficient of electromagnetic power is calculated by damping torque method, and the influence of power system stabilizer on ultra-low frequency oscillation is analyzed. Finally, the conclusion is verified by simulation in IEEE 4-machine 2-zone system.