Condition　of　insulation　materials　can　be　obtained　by　analyzing　partial　discharge　(PD)　characteristics　since　features　of　PD　are　closely　related　to　it.　In　this　paper,　electrical　deterioration　caused　by　partial　discharge　of　oil-paper　insulation　under　long-term　AC　voltage　is　studied.　Feature　characteristics　of　PD,　such　as　phase-resolved　partial　discharge　analysis　and　equivalent　time-frequency,　were　recorded　during　the　test　process.　Electrical　deterioration　is　divided　into　four　stages:　initial,　develop,　stagnation　and　erupt　stage.　Four　stages　are　separated　by　pulse　repetition　rate　and　discharge　magnitude　of　PD.　In　different　deterioration　stage,　statistical　characteristics　of　PD　change　significantly.　Surface　properties　of　pressboard　in　different　deterioration　stage　were　studied.　Test　results　indicate　that　the　depolarization　of　cellulose　in　pressboard　is　the　main　reason　for　development　of　partial　discharge,　and　discharges　occur　in　different　position　of　pressboard　in　different　deterioration　stages.