Abstract -- Power Conversion System (PCS) is the critical interface connecting energy storage devices and the utility grid. In this paper, the research is developed on two-stage PCS consists of DC/DC converter regulating battery current and DC/AC converter regulating voltage of DC bus. The positivesequence and negative-sequence impedance of the two-stage PCS at PCC (Point of Common Coupling) is modeled through harmonic linearized method with mirror frequency coupling effect considered. The validity of the impedance model obtained is verified through simulation, simulation results indicates that the proposed approach achieved a desirable accuracy .With the obtained impedance, stability analysis of PCS against grid could be implemented through Nyquist criteria, the analysis proves to be right by simulation.