Aiming　at　the　complexity　of　dielectric　relaxation　response　of　transformer　oil-paper　insulation,　a　hybrid　equivalent　circuit　new　model　was　established　to　characterize　the　relaxation　process　of　oil-paper　insulation　system　according　to　the　characteristics　of　oil-paper　insulation.　According　to　theory　analysis　of　hybrid　equivalent　circuit,　formula　of　the　hybrid　equivalent　circuit　parameters　and　the　recovery　voltage　were　deduced.　Test　data　of　a　transformer　recovery　voltage　was　adopted　to　verify　the　method,　and　then　the　test　data　of　a　transformer　before　and　after　being　maintained　was　used　to　diagnose　transformer　oil-paper　insulation　system　based　on　hybrid　equivalent　circuit.　The　results　show　that:　the　return　voltage　curve　which　is　gained　by　the　hybrid　equivalent　circuit　method　inosculates　very　well　with　those　gained　by　the　field　measurements,　and　the　hybrid　equivalent　circuit　can　diagnose　aging　of　transformer　oil-paper　insulation　dielectric　accurately.　©　2016,　Editorial　Department　of　Electric　Machines　and　Control.　All　right　reserved.