Abstract: With the increasing proportion of distributed generations in distribution networks, harmonic resonance problems caused by interactions between grid-tied inverters and weak grid are becoming increasingly prominent. Small-signal modeling of inverters is the foundation of harmonic stability analysis of the inverter-weak grid coupling system. Based on the small signal model of typical grid-connected inverters in dq coordinate, a simplified impedance modeling method has been proposed in this paper. In the modeling process, matrix operations were reduced and the coupling terms between d and q channels were categorized into negligible and non-negligible parts, so that the connections among inverter impedance terms and the physical segments of inverters can be highlighted to strengthen physical interpretations of dq impedance. In addition, the concepts of active power admittance and reactive power admittance were introduced to demonstrate the influence mechanism of stable operating point’s variations on system stability through phase locked loop（PLL）. Finally, the theoretical analysis in frequency domain and simulation results in time domain were provided to prove that the proposed simplified impedance model can be used to analyze the resonance mechanism and predict the stability of the inverter-weak grid coupling system.