Owing to the impedance interaction between the inverter and grid, grid impedance will adversely affect the stability of the grid-connected inverter system under weak grid conditions, which can easily cause system resonance. Firstly, the mechanism and characteristics of system resonance are analyzed through impedance modeling and stability analysis. With the increase of the output power for inverter, PLL bandwidth (BWPLL), or the decrease of short-circuit ratio (SCR) of the grid, the grid-connected inverter system tends to instability. In order to improve the adaptability of the inverter in a weak grid, an impedance-based resonance suppression method is proposed, which consists of virtual impedance compensation based on feedforward loop and current loop impedance reshaping. Then, the impedance model of the inverter with the proposed resonance suppression control method is established considering the effects of frequency coupling. The comparative analysis proves that the proposed control method improves the stability of the system. Finally, the effectiveness of the proposed control strategy is verified through experiments.