The grid impedance ratio X/R (GIR) and the short circuit ratio (SCR) are the indexes that widely applied to the stability analysis of power systems integrated with voltage source converters (VSC). Based on the assumption that GIR is large enough, the impacts of grid resistances are neglected in most studies. In this brief, the impact mechanisms of grid impedance are analyzed by single-input and single-output (SISO) transfer functions under different GIR conditions. It is found that the decrease of GIR has both “enhancement effect” and “reduction effect” on SISO transfer function magnitudes, while the “reduction effect” is stronger than the “enhancement effect”. When $\mathrm {GIR} \in [f_{0}/f_{\mathrm{ pll}}, \infty$ ), the interaction behaves between the grid impedance and phase-locked loop (PLL), and it mainly has impacts on the magnitude-frequency curve. When $\mathrm {GIR} \in [0, f_{0}/f_{\mathrm{ pll}}$ ), the interaction behaves among the phase of grid impedance, PLL and current loop, and it mainly has impacts on the phase-frequency curve. Simulations are carried on MATLAB to verify the effectiveness of the proposed analysis.