Abstract: Virtual inertia control of doubly-fed induction generator (DFIG) can improve the frequency stability, and its transient response characteristic may have impact on the transient stability of power system. Based on the analysis of transient active power output characteristics of DFIG under different depth of voltage sags, the transient active power output of DFIG is determined by virtual inertia control. Furthermore, the influence of virtual inertia control of DFIG on transient stability of power grid is investigated by using the extended equal area criterion (EEAC). The time domain simulation results indicate that DFIG with virtual inertia control in S group can improve transient stability of power grid, and DFIG with virtual inertia control in R group can deteriorate transient stability of power grid if low voltage ride through control of DFIG is not triggered after failure. The results give some suggestions on the application and optimization of virtual inertia control of DFIG.