Photovoltaic (PV) generation system can dynamically control the active power and reactive power injected into the grid to change the dynamic characteristics of the synchronous machine power system, and effectively suppress the power system power oscillation phenomenon. In order to reveal the mechanism of PV to suppress power system oscillations, this paper describes the active and reactive power output characteristics of PV on the electromechanical time scale through two controlled current sources. Based on this, the active power distribution model of the grid is established when PV participates in oscillation suppression. Then, the electrical torque method was used to analyze the physical mechanism of PV affecting the inertia, damping capacity, and synchronization characteristics of the synchronous machine system under the oscillation suppression mode and its influence law. The research shows that: If PV dynamically adjusts the active and reactive power output according to the change trend of the characteristic quantities such as speed, frequency, power and power angle during the oscillation process, the grid's damping capacity, synchronization capacity and inertia will be equivalently changed. Therefore, PV can actively adjust the oscillation's amplitude, period and attenuation speed based on the grid operation requirements. The simulation results verify the correctness of PV suppressing grid oscillations.