Abstract: The asymmetric dq-axis structure or parameters of the controller in the grid-connected inverter will cause the coupling between two disturbed frequency components of the system in the static frame.This coupling factor will affect the stability of the power system.This paper analyzes the mechanism of frequency coupling,and derives the equivalent admittance of inverter under the two coupling frequency components.By using the two equivalent admittances,the stability of the gridconnected inverter system can be judged by the Nyquist criterion.Compared with the approaches using the generalized Nyquist criterion,the stability judgement process of the proposed method is simpler.By using this method,it is convenient to obtain the phase margin or gain margin of the system and guide the controller design.Finally,the correctness of the stability judgement results is verified through experiments.