Grid-forming VSCs based on Virtual Synchronous Generator (VSG) control strategy provide primary frequency regulation and voltage support for weak grids or VSC-dominated grids. Analyzing the transient stability of VSG-VSCs under large disturbances is useful for maintaining the security of the grid. However, different control strategies in grid-forming converters will cause the VSC to operate at different transient processes, and this topic is little studied. In this paper, the transient processes of the VSG-VSC are theoretically explained. In addition, the current unsaturation mode and current saturation mode of the VSC, and the impacts of Q-V controller on transient process under severe voltage drops are elaborately studied. In particular, it is shown that the change of controller will change the transient process of the VSC. Simulations verify the validity of the transient stability analysis.