In a new energy grid-connected inverter it is easy to induce wide-band oscillation under a weak power grid. To analyze the occurrence mechanism of oscillation instability, the impedance model of a three-phase LCL grid-connected inverter considering the influence of phase-locked loop is established based on the complex vector transfer function method. Then the mechanism of frequency coupling is analyzed. The equivalent output impedance model of the interaction between grid-connected inverter system and grid impedance is derived, and the influence of phase-locked loop on the stability of the grid-connected inverter is analyzed using, an impedance stability criterion. To limit the adverse effect of the phase-locked loop on the stability of the grid-connected inverter system, an improved control strategy of disturbance voltage feedforward compensation is proposed. Finally, a Matlab/Simulink model is built to verify the accuracy of the stability judgment based on the established impedance model and the effectiveness of the improved control strategy. This work is supported by the Science and Technology Plan of Guizhou Province (No. [2018]5615).