To　assess　the　oil-paper　insulation　of　transformer,　it　is　necessary　to　establish　the　equivalent　circuit　for　it　regardless　of　using　the　time　domain　methods　or　frequency　domain　methods.　Based　on　the　principle　of　dielectric　polarization,　the　extended　Debye　model　is　always　adopted　for　the　equivalent　circuit　of　oil-paper　insulation　in　transformer　in　most　cases;　however,　it　is　usually　difficult　to　determine　the　number　of　relaxation　mechanisms　in　the　model.　This　paper　presents　a　method　to　determine　it　by　the　time　domain　dielectric　spectra,　namely,　conduct　differential　operation　for　the　depolarizing　current　of　transformer＇s　oil-paper　insulation　to　obtain　the　time　domain　dielectric　spectra,　and　the　number　of　spectrum　peaks　is　equal　to　that　of　relaxation　mechanisms.　Thereby,　the　extended　Debye　model　of　the　equivalent　circuit　of　the　oil-paper　insulation　of　transformer　can　be　built.　This　paper　describes　the　analytical　procedures　of　the　time　domain　dielectric　spectra　method　in　detail　and　presents　examples　to　validate　its　feasibility　and　rationality　in　determining　the　equivalent　circuit　model　of　transformer＇s　oil-paper　insulation.　©　(2013)　Trans　Tech　Publications,　Switzerland.