In recent years, new energy power generation has gradually become a general trend. however, with the large-scale access of new energy generating units, the systems on both sides of the converter show the characteristics of weak AC system, at this time, the traditional vector control strategy will be affected by phase-locked loop parameters, resulting in system instability. To solve this problem, this paper first establishes a small signal model for VSC in weak AC system, and analyzes the influence mechanism of AC system short-circuit ratio and phase-locked loop parameters on the overall system stability under vector control strategy. Then, according to the power synchronization control principle, a new improved phase-locked loop control structure is proposed, and the corresponding control strategy is designed. The improved phase-locked loop can theoretically improve the system stability when the weak AC system is connected to the VSC. Finally, the improved PLL control structure is verified by small signal model analysis and Simulink simulation. The results show that in the weak AC system, the improved PLL control structure can effectively track the voltage phase of the common coupling point, and can effectively improve the system stability and stabilize the active and reactive power output of the system. Compared with the traditional PLL structure, it has obvious application advantages and value.