The　reply　voltage　method　based　on　the　theory　of　dielectric　response　is　effective　in　non-destructive　diagnosis　of　transformer　insulation　condition.　The　polarization　equivalent　circuit　model　of　transformer　oil-paper　insulation,　especially　the　calculation　of　its　parameters,　plays　a　key　role　in　the　process　of　diagnosis.　However,　the　popular　extended　Debye　model　is　not　able　to　determine　the　number　of　polarization　branches.　In　this　paper,　the　extended　Debye　model　is　solved　by　setting　different　number　of　polarization　branches.　Because　the　worse　the　insulation,　the　more　the　polarized　branches,　the　number　of　polarization　branches　of　the　transformer　is　assumed　to　be　7　for　poor　insulation　condition,　and　5　for　good　insulation　condition.　Subsequently,　Explores　the　relationship　among　different　number　of　polarization　branch　is　obtained.　Combining　the　heuristic　approach　of　branch,　a　characteristic　quantity,　the　time　constant　of　polarization　branch,　is　suggested　for　evaluating　the　aging　condition　of　transformer　oil-paper　insulation.　©,　2015,　Xi＇an　High　Voltage　Apparatus　Research　Institute.　All　right　reserved.