A　judgment　method　for　the　polarization　branches　number　of　the　equivalent　circuit　of　oil-paper　insulation　equipment　is　proposed.　The　differential　dielectric　spectroscopy　method　is　applied　to　gradually　decomposing　the　measured　return　voltage　curves.　In　this　way,　all　the　sub-lines　which　are　implicit　in　the　internal　relaxation　response　of　the　return　voltage　curves　could　be　presented.　Then　the　polarization　branches　number　of　extended　Debye　model　is　judged　by　the　number　of　sub-lines.　Results　show　that　the　more　aging　seriousness　of　the　transformer　oil-paper　insulation　is,　the　more　numbers　of　the　polarization　branches　and　the　higher　relaxation　polarization　will　be.　The　value　of　the　relaxation　polarization　is　small　when　the　oil-paper　insulation　is　in　good　condition.　Otherwise,　the　value　is　greater.　The　proposed　method　is　an　effective　way　to　evaluate　the　oil-paper　aging　using　the　equivalent　circuit　of　relaxation　response.　©　2016,　The　editorial　office　of　Transaction　of　China　Electrotechnical　Society.　All　right　reserved.