Moisture　is　a　strong　polar　molecule,　and　it　could　change　the　relaxation　time　of　dielectric　and　accordingly　affect　the　state　of　transformer　insulation.　So　it　is　of　great　significance　to　analyze　the　transformer＇s　aging　and　damp　condition　with　the　method　of　diagnosing　the　micro　water　content　of　the　transformer　accurately.　By　applying　the　dielectric　response　return　voltage　method,　combining　with　the　characteristics　of　the　return　voltage　polarization　spectrum　by　field　test　to　discuss　the　relationship　between　the　micro　water　content　and　the　characteristics　of　the　return　voltage　polarization　spectrum　further,　we　have　come　up　with　an　conclusion　that　the　smaller　the　dominant　time　constant　is,　the　more　water　the　transformer　contains;　the　larger　the　initial　slope　is,　the　more　water　the　transformer　contains.　At　the　same　time,　with　the　increase　of　water　content　in　the　transformer　the　maximum　value　of　return　voltage　will　also　be　increased.　So　we　can　diagnose　the　micro　water　content　of　the　transformer　by　analyzing　the　characteristics　of　the　return　voltage　polarization　spectrum,　so　as　to　assess　the　aging　and　damp　state　of　oil-paper　insulation　transformer　effectively.