Abstract: Aiming at the problem of poor transient stability and unclear instability mechanism of the voltage source converter（VSC）that adopts the grid-forming control strategy（take the virtual synchronous generator control as an example） with a current limiting loop, this paper starts with the transient stability analysis of the traditional synchronous generator, and qualitatively analyzes the transient process of the grid-forming VSC. Through the nonlinear differential equation analysis method based on the phase portrait,the quantitative analysis of the transient instability mechanism of the grid-forming VSC is implemented. Moreover, a new synchronization method（called hybrid synchronization control） that combines the swing equation of the traditional synchronous generator and the phase-locked loop is proposed, which makes the VSC not only has the ability of frequency response, but also greatly improve its transient stability margin. Finally, the simulation model built in MATLAB/Simulink verifies the accuracy of the analysis method and the effectiveness of the proposed control scheme.