Under weak grid conditions, the system generates resonance due to the coupling between parallel inverters and between inverters and the grid. At the same time, the grid impedance in the actual system and the line impedance from the inverter to the grid connection point cause the resonant point to shift, aggravating the system resonance instability. Under the background, a multi-inverter parallel mathematical model considering grid impedance and line impedance is established on the basis of a single grid-connected inverter firstly. Then, the resonance formation mechanism of the parallel system under weak grid is discussed and the influence of grid and line impedance on system resonance is analyzed. Meanwhile, a resonance suppression method is proposed by combining the full feedforward of the incoming current and the virtual admittance in parallel at the PCC point. Finally, three parallel system simulation models of T-type three-level inverters based on LCL filters are built in Simulink. The simulation results show that this method can effectively suppress the inherent resonance of the LCL grid-connected inverter and the resonance caused by the weak grid. In addition, the method can also effectively improve the system stability and enhance the robustness of the multi-machine parallel system to changes in line impedance and grid impedance.