ThispaperputsforwardanewfaultdiagnosismodelbasedonPetrinet.Inthisnewfaultdiagnosismethod,anassociatedtreeisfirstdefinedtodescribethelogicaccordingtotheprotectionsandcircuitbreakers.AfterthataPetrinetdiagnosismodelcomesintobeingandtheprecisediagnosisofpowergridelementscanberealizedatthesametime.Thesimulationresultsgiveninthispaperillustratethatthisfaultdiagnosismethodcanmakearapiddiagnosisincomplexfaultsituationandgiveaccurateresultstopowergridfaultdiagnosis.