Based　on　the　study　of　dielectric　response　theory　and　equivalent　circuit　of　oil-paper　insulation　medium,　this　paper　deduces　the　exponential　decay　type　return　voltage　function　model.It　also　proposes　that　rhe　function　can　be　resolved　with　the　application　of　optimization　algorithm.　Finally,　the　paper　takes　a　return　voltage　curve　which　is　tested　by　the　oil-immersed　transformer　for　example.Fitting　results　verify　the　accuracy　of　the　calculation.　Further　more,it　also　explains　that　return　voltage　function　model　is　reliable　and　practical.　Therefore,the　model　plays　a　crucial　role　in　the　follow-up　study　of　topological　structure　of　equivalent　circuit　of　oil-paper　insulation.　©　(2014)　Trans　Tech　Publications,　Switzerland.