Abstract: In order to solve the problem of power system oscillation caused by power electronic converter, this paper took the rotor-side converter of doubly-fed induction generator as an example, derived the inertia expression of the converter and explanation of physical meaning of inertia, meanwhile compared it with the traditional inertia of power system: rotor inertia and electromagnetic inertia of inductance and capacitance, and a unified differential equation describing the inertial elements of power system was obtained. At the same time, the criterion of converter instability was deduced according to the differential equation. Finally, the conclusions were verified by PSCAD/EMDTC simulation. The inertia of power electronic converter proposed in this paper provides a new idea for studying the stability of power electronized power systems.