The　construction　of　equivalent　circuit　model　of　oil-paper　insulation　for　transformer　is　critical　to　study　the　evaluation　of　the　aging　status　of　oil-paper　insulation,　and　the　unique　identification　of　the　equivalent　circuit　parameters　is　a　challenging　issue　in　this　part.　In　view　of　the　lack　of　non-uniqueness　in　the　methods　for　identifying　equivalent　circuit　parameters,　this　paper　presents　a　new　method　for　calculating　the　parameters　of　extended　Debye　equivalent　circuit　of　oil-paper　insulation　in　conjunction　with　the　three　time　domain　differential　curve.　Firstly,　the　unique　identification　of　the　polarization　branch　number,　time　constant　and　relaxation　coefficient　of　the　equivalent　circuit　is　realized　by　using　the　uniqueness　of　the　terminal　peak　point　of　the　three　time　differential　time　domain　differential　curve　of　depolarized　current.　Secondly,　according　to　the　theory　of　equivalent　circuit　and　basic　circuit,　the　function　of　the　parameters　of　each　branch,　time　constant　and　relaxation　coefficient　is　derived,　and　the　resistance　and　capacitance　value　of　the　equivalent　branch　of　each　polarization　are　calculated.　Finally,　the　reliability　and　accuracy　of　this　method　for　identifying　the　equivalent　circuit　parameters　of　oil-paper　insulation　is　verified　by　an　application　case.　©　2018,　Science　Press.　All　right　reserved.