To enhance the support capacity of renewable energy generation equipment for the power grid, grid-forming control has received extensive attention, among which virtual synchronous generator (VSG) has become a research frontier and has been applied in engineering demonstration. Voltage source converter (VSC) with VSG as the synchronization link can be divided into voltage and current dual loop control and direct voltage control according to whether there is a current control loop in the structure. The difference in the two control structures has a significant impact on its transient characteristics. To study the difference in transient characteristics between two kinds of VSC, the transient models are established separately based on “power excitation-internal voltage response” modeling concept, and the formation mechanism of internal voltage and transient characteristics are comparatively analyzed. Since the VSG simulates the operation characteristics of the synchronous machine, the equivalent inertia and equivalent damping of the VSC are analytically obtained at the electromechanical scale, and their transient characteristics are compared. It is found that the equivalent inertia / damping of VSC with direct voltage control is constant, while the equivalent inertia / damping of VSC with voltage and current dual loop control has time-varying characteristics and is numerically smaller than that of direct voltage control. Finally, the correctness of the theoretical analysis is verified by electromagnetic transient simulation.