Different　control　methods　for　new　energy　power　generation　access　to　systems　will　bring　different　influences　to　the　inertia　of　system.　The　real-time　calculation　method　to　calculate　power　shortage　based　on　estimated　inertia　is　proposed.　Firstly,　the　shortcomings　of　the　existing　under-frequency　load　shedding　equipment　for　calculating　power　shortage　are　analyzed.　In　view　of　these　shortcomings,　the　influence　of　voltage　deviation　on　active　power　balance　is　studied,　and　the　quantization　algorithm　is　proposed.　According　to　the　response　information　of　load　after　disturbance,　a　real-time　method　to　recognize　system　inertia　is　proposed.　Then,　the　real-time　calculation　method　to　calculate　power　shortage　based　on　estimated　inertia　is　proposed.　The　simulative　results　of　islanding　system　in　IEEE　39-bus　system　verify　the　influence　of　the　voltage　offset　on　the　calculation　of　power　shortage,　and　the　veracity　and　validity　of　recognizing　system　inertia　and　calculating　power　shortage　in　real-time.　Compared　with　the　traditional　round-by-round　load　shedding　schemes,　the　proposed　scheme　has　relatively　small　amount　of　load　shedding　and　dynamic　frequency　offset　and　stronger　ability　of　frequency　recovery.　©　2019,　Electric　Power　Automation　Equipment　Press.　All　right　reserved.