Due to the existence of grid impedance in a weak grid, the application of voltage feedforward control can adversely affect the stability of the inverter system. In order to preserve the advantages of voltage feedforward control, according to the impedance stability analysis method, the problems in adopting voltage feedforward control under grid impedance variation in weak grids are studied. Instability mechanism and unstable region are also presented. Based on the analysis, a phase lead compensation method is proposed to reduce the unstable region caused by voltage feedforward control, increase the phase angle of the inverter output impedance, and safeguard strong robustness to the system when the grid impedance varies. Finally, the simulation experiment is tested in Matlab/Simulink. Simulation results verify the effectiveness and feasibility of the proposed method.