An　active　power　filter　(APF)-type　active　damper　is　proposed　to　mitigate　the　high-frequency　oscillations　in　the　grid-tied　inverter　system.　This　damper　follows　the　rules　of　APF　by　sampling　the　harmonic　current　of　the　original　system,　which　will　introduce　a　coupling　effect　between　the　damper　and　the　original　inverter.　This　is　different　from　the　traditional　active　damper　which　only　extracts　the　voltage　signal　of　the　point　of　common　coupling　(PCC),　thus　the　original　inverter　and　the　damper　are　independent.　When　extracting　the　harmonic　current,　the　coupling　effect　poses　a　great　challenge　to　the　impedance　modeling　and　stability　analysis.　This　paper　first　establishes　the　dq-impedance　model　of　the　active　damper　with　the　coupling　effect　included.　Then　the　harmonic　current　mitigation　control　is　equivalent　into　an　additional　damping　impedance　that　is　decided　by　the　original　inverter　and　the　capacitor.　Later,　the　stability　effect　of　the　active　damper　is　analyzed　and　verified　via　experimental　results.　It　is　finally　concluded　that　the　APF　-type　active　damper　can　provide　a　well　damping　effect　to　improve　the　system　stability　by　increasing　the　phase　margin　at　resonant　frequency　area.　©　2022　IEEE.