The　virtual　synchronous　generator　(VSG)　is　a　control　scheme　applied　to　the　grid-tied　inverter　to　increase　system　inertia　by　imitating　the　rotor　inertia　of　synchronous　generator　(SG).　However,　due　to　the　imitation　on　rotor　inertia　VSG　control　may　lead　to　the　power　and　frequency　oscillation　after　a　change　or　disturbance,　which　decreases　the　reliability　of　inverters.　In　this　paper,　the　process　of　active　power　and　frequency　oscillation　is　divided　into　four　stages　and　analyzed　in　detail.　Then　the　optimal　criterion　of　virtual　angular　acceleration　in　the　four　stages　are　provided　to　improve　the　transient　response　of　power　and　frequency.　An　optimal　control　method　of　virtual　angular　acceleration　is　proposed　according　to　the　optimal　criterion.　Simulation　and　experiment　results　are　presented　to　validate　the　effectiveness　of　the　proposed　method.