Under a weak grid, the grid impedance will affect the synchronization stability of the grid-connected converter. However, existing research mainly focuses on the impact of grid impedance, that is, grid strength, on stability, and rarely involves the impedance ratio (R/X). Based on this, this paper establishes a dynamic coupling model involving grid impedance, current control loop and phase-locked loop. On the basis of the built model, the influence of grid impedance ratio on synchronous stability is revealed, and its instability mechanism is clarified from a physical perspective. Finally, the simulation verified the correctness of the theoretical analysis.