This paper presents a modified dq impedance model of the three-phase voltage source grid-connected inverter (GCI)-grid system considering coupling effect between GCI part and grid part for small-signal stability analysis. Steady-state terminal voltage and current information of the GCI part which are necessary in conventional dq impedance modelling methods are calculated based on grid parameters, i.e., grid voltage and grid impedance, and current references of current-controlled GCI or power references of power controlled GCI. In addition, effects of dynamics of grid voltage and phase-locked loop (PLL) on dq impedance characteristics of grid part are also investigated, which enables the modified dq impedance model of grid part to relate with that of GCI part. The modified dq impedance model of the GCI part which takes the effects of grid voltage and grid impedance into account need not know steady-state terminal voltage and current information. Simulation results are given to validate correctness of the proposed dq impedance model of the GCI-grid system, and effectiveness of the dq impedance model for stability analysis.