When the grid-connected inverters (GCI) connect with the weak grid, they will suffer from synchronization stability problems. The instability issues will limit the development of the renewable energy system. In this paper, an intuitive explanation of the coupling between the phase-locked loop (PLL) and weak grid is provided. Then, an improved PLL structure is adopted to increase the synchronization stability based on the coupling mechanism, which equivalently parallels a resistance with the grid impedance and modifies the q-q-axis impedance. Finally, experimental results further validate the effectiveness of the improved PLL structure.