The　study　on　the　inner　relationship　between　oil　paper　insulation　equivalent　circuit　parameters　and　insulation　aging　mechanism　is　less　and　immature,　and　the　relationship　between　the　equivalent　circuit　parameters　and　oil　paper　insulation　aging　was　analyzed　in　this　paper.The　influence　of　the　equivalent　circuit　parameters　on　oil　paper　insulation　aging　of　transformer　was　qualitatively　studied.　It　is　found　that　key　parameter　for　evaluating　insulation　condition　of　power　transformers,　average　relaxation　time　constant　of　equivalent　circuit,　was　almost　not　affected　by　the　geometry　structure　of　the　insulation　system.　Furthermore,　the　criterion　was　obtained　to　diagnose　insulation　condition　of　power　transformers　by　using　the　average　relaxation　time　constant　of　equivalent　circuit.　Meanwhile,　the　preliminary　quantitative　criterion　was　obtained　by　analyzing　a　lot　of　examples　which　were　given　to　verify　that　the　criterion　is　correct.　The　analysis　results　show　that　the　method　provides　new　ideas　to　diagnose　insulation　condition　of　power　transformers.　©　2017,　Harbin　University　of　Science　and　Technology　Publication.　All　right　reserved.