As the penetration of the distributed energy source in the grid is becoming higher and higher, the grid is increasingly showing the characteristics of weak grid due to some factors like the longer transmission lines and distribution transformer leakage inductance. The proportion feedforward system is widely employed to reduce the current steady-state error in weak grid conditions. The phase margin will be greatly reduced, which affects the stability of grid-connected inverter due to the introduction of the positive feedback loop associated with the grid impedance. Taking the three-phase LCL grid-connected inverter as an example, the influence of the grid impedance on the stability of grid-connected inverter is analyzed by the Bode diagram, and the control strategy that connects a complex filter in series in grid-connected voltage proportional feedforward loop is proposed. The analysis shows that the strategy can improve the stability of grid-connected inverter in a weak grid. Finally, a 5 kW three phase grid-connected inverter simulation model is built to verify the effectiveness of the proposed control strategy. This work is supported by National Natural Science Foundation of China (No. 61703144) and Key Science and Technology Project of Colleges and Universities of Henan Education Department (No. 17A470010).