Abstract: A small-signal model of synchronous generators（SG）and VSG was first established. Based on the same control theory,the effects of SG and VSG’s frequency modulation capability and inertia level on the low-frequency oscillation mode of the system were analyzed. The effect differences and the reasons for the difference of SG and VSG on low-frequency oscillation were studied. The research results show that the virtual impedance control link of VSG is the key factor that makes VSG and SG inconsistent in low frequency oscillation characteristics. When the virtual impedance is small, the influence of VSG on low frequency oscillation mode is similar to SG.