To　analyze　the　grid　integration　effect　of　a　virtual　synchronous　generator　(VSG),　this　paper　proposes　a　simple　power　system　modeling　method.　It　is　shown　that　for　a　specific　disturbance,　by　simplifying　the　original　network　to　a　radical　n　generators　single-load　model,　its　state-space　small-signal　model　can　be　obtained　using　a　unified　formula,　as　long　as　the　single　generator　infinite　bus　model　of　each　generator　is　given.　Case　studies　based　on　the　IEEE　9-bus　test　system　as　well　as　the　New　York　New　England　test　system　show　high　efficiency　of　the　proposed　modeling　to　represent　slow　power　and　frequency　deviations.　Modal　analyses　give　insight　into　how　system　poles　are　synthesized　by　individual　generator　poles,　and　how　different　designs　of　VSGs　affect　the　system　oscillation　modes.　Potential　errors　due　to　the　topology　simplification　and　the　measures　to　avoid　them　are　discussed.　The　time-domain　results　and　modal　analyses　highlight　some　important　features　of　VSG,　which　are　overlooked　in　previous　studies.