It has been reported that synchronization instability of grid-tied power converters may be activated when severe grid voltage sags occur, since the post-fault equivalent grid impedance becomes pretty considerable and thus makes the terminal voltage of power converters quite weak. This paper develops a simplified reduced-order large-signal model to describe the dynamic synchronization characteristics of grid-tied power converters. By considering the role of the grid impedance and analyzing spatial vector tracking relationships, the synchronization principle of power converters is elaborated. The effects of the power grid parameters and control parameters, including residual voltage, post-fault grid impedance, PLL parameters, and current references, on the synchronization stability are analyzed, and also further verified by simulation and experimental results.