Voltage source converters (VSCs) play an important role in grid-connected renewable system. However, the synchronization stability of VSCs is affected by many factors, such as grid impedance, controller parameters, and system disturbance. It is found that system is prone to lose transient synchronization stability when large series compensation is deployed. The large frequency disturbance and reduction of stability region account for the synchronization instability. Traditional methods, such as redesigning PI parameters, and deploying band-pass filter, can improve synchronization stability, but they highly depend on system detailed models and usually sacrifice original control performance. This paper proposes a hybrid optimization method by combing PI controller and fuzzy controller to improve synchronization stability. The fuzzy controller is designed to limit transient frequency deviation and the PI controller is deployed to obtain better static performance. The simulation results have verified that the method can effectively improve system stability and with higher control performance than traditional methods.