With　concerns　about　the　decline　of　the　original　inertia　of　power　system,　virtual　synchronous　generator　(VSG)　is　introduced　and　becomes　a　research　hotspot　rapidly.　To　analysis　the　impedance　characteristic　of　VSG　and　grid-tied　system　stability,　this　paper　firstly　proposes　a　novel　outer　control　loop　of　power　for　the　VSG.　Combined　with　the　classical　inner-current　loop,　fast　current　regulation　and　over-current　protection　and　vector　control　can　be　realized.　Afterwards,　small-signal　impedance　models　of　VSG　as　well　as　inner-current　and　outer-power　loops　in　the　synchronous　reference　(d-q)　frame　are　derived.　As　the　d-axis　of　the　d-q　frame　of　VSG　varies　with　the　command　reference　and　doesn＇t　always　coincide　with　the　system　voltage　vector　during　steady　state,　an　impedance　transformation　matrix　(ITM)　is　introduced,　through　which　all　the　impedances　can　be　reconstructed　to　a　unified　d-q　frame　for　the　sake　of　comparison　and　analysis.　On　the　basis　of　the　work　above,　the　impedance　characteristics　and　related　influence　factors　like　inertia,　damping　factor　are　analyzed.　Results　show　that:　1)　Inertia　and　damping　factors　have　little　effects　on　the　impedance　characteristic;　2)　Compared　with　PLL-based　inverter　impedance,　the　impedance　characteristic　of　VSG-based　inverter　is　much　better　for　system　stability;　3)　At　the　unity　power　factor　working　point,　the　resistance　part　of　is　positive,　being　of　importance　for　suppressing　sub-synchronous　oscillation　caused　by　negative　resistance.　Finally,　simulation　of　data　on　MATLAB　/Simulink　is　built　to　testify　the　validity　of　the　small-signal　impedance　models.　©　2022　IEEE.