Abstract: The grid-forming converter has voltage source characteristics and has good stability under high-impedance weak grid, but it may be unstable in low-impedance strong grid. Aiming at the stability problem of grid-forming converter connected to low-impedance grid, that paper first establishes the state space model of grid-forming converter under low impedance, and combines the characteristic root trajectory and participation factor to analyze the dominant mode and stability trend of the system under different grid strength and different control parameters. On the basis of the state space model, the dynamic model of the grid converter connected to the AC power grid is established to characterize the dynamic characteristics of the internal potential-inertia, damping component, and synchronous component. The stability mechanism of grid-forming converter under low impedance is revealed by analyzing the evolution law of damping component and synchronous component of grid-forming converter with the change of grid strength and control parameters. The analysis results show that the terminal voltage control loop of the grid-forming converter will introduce significant additional negative damping components under low impedance, which reduces the stability of the system. By increasing the bandwidth of the terminal voltage control loop, the negative additional damping can be reduced and the stability can be improved. At the same time, reducing the inertia coefficient and increasing the damping coefficient can also increase the damping component and improve the stability.