To　provide　inertia　support　for　the　grid,　virtual　synchronous　generator　(VSG)　control　of　inverter-interfaced　distributed　generators　(IIDGs)　becomes　a　focus　of　worldwide　attention.　However,　a　VSG-based　IIDG　behaves　differently　in　the　grid-connected　(GC)　mode,　the　islanded-single-DG　mode,　and　the　islanded-multi-DG　mode,　whereas　the　mathematical　and　physical　interpretations　of　this　phenomenon　are　not　well　studied.　In　this　article,　we　propose　a　unified　modeling　method　of　VSG-based　IIDG　to　analyze　its　different　dynamic　performance　in　each　operation　mode.　The　proposed　unified　formulas　can　obtain　the　state-space　models　of　the　islanded-single-DG　and　islanded-multi-DG　modes　from　that　of　GC　mode　for　any　VSG　control　method.　With　the　obtained　models,　for　several　different　types　of　VSG　control　in　different　operation　modes,　we　analyze　the　distribution　and　sensitivity　of　the　closed-loop　poles　and　investigate　the　step　responses　both　analytically　and　experimentally.　These　analyses　reveal　the　intrinsic　differences　and　correlations　of　the　dynamics　of　VSG-based　IIDG　between　each　operation　mode.　These　intrinsic　features　are　valid　independent　of　the　applied　VSG　control　scheme;　thus,　a　test　method　to　evaluate　the　parameters　and　performance　of　an　unknown　IIDG　is　derived.　The　findings　of　this　article　provide　important　instructions　for　engineers　to　model　and　design　and　test　multi-operation-mode　distributed　generators.