Return　voltage　measurement　based　on　the　dielectric　response　theory　is　a　non-destructive　diagnostic　method,　which　can　detect　variation　of　dielectric　parameters　of　insulation　material　and　assess　water　content　in　transformer　insulation　by　polarization　properties　availably.　Consequently,　the　principle　of　assessing　water　content　by　return　voltage　method　in　detail　is　expounded,　and　an　experimental　study　is　conducted　to　analyze　the　relation　between　characteristic　quantity　of　return　voltage　and　moisture　in　oil-paper　insulation　transformer.　The　experimental　results　are　analyzed　and　show　that　dominant　time　constant　can　be　connected　with　its　moisture　content.　Meanwhile,　repairing　quality　of　a　transformer　with　220　kV　is　verified　by　the　method,　showing　that　the　method　is　effective　for　diagnosing　oil-paper　insulation　condition　of　transformer.　The　influences　of　polarization　spectrum　are　also　researched,　and　the　results　show　that,　at　the　lower　return　voltage　of　transformer,　environmental　effect　is　significant.　Therefore,　it　is　important　to　remove　all　kinds　of　interference　in　order　to　obtain　accurate　measurement　results.　Moisture　in　transformer　will　accelerate　ageing　of　insulation　paper,　therefore,　it　is　imperative　to　diagnose　water　content　of　solid　insulation　for　assuring　a　normal　operation　of　power　transformers　and　delaying　insulation　ageing.　The　moisture　of　dielectric　will　change　the　microstructure　of　the　dielectric　molecules,　and　therefore　it　affects　the　conductivity　and　polarization.　Return　voltage　measurement　based　on　dielectric　response　theory　is　a　non-destructive　diagnostic　method,　which　can　detect　variation　of　dielectric　parameters　of　insulation　material　and　assess　water　content　in　transformer　insulation　by　polarization　properties　availably.　This　paper　expounds　the　principle　of　assessing　water　content　by　return　voltage　method　in　detail,　an　experimental　study　was　conducted　to　analyse　the　relation　between　characteristic　quantity　of　return　voltage　and　moisture　in　oil-paper　insulation　transformer.　The　experimental　results　are　also　detailed　and　analyzed,　which　show　that　dominant　time　constant　can　be　connected　with　its　moisture　content.　Meanwhile,　repairing　quality　of　a　transformer　with　220　kV　was　verified　by　the　method,　it　shows　that　the　method　is　effective　for　diagnosing　oil-paper　insulation　condition　of　transformer.　The　influences　of　polarization　spectrum　are　also　researched;　the　result　show　that　at　the　lower　return　voltage　of　transformer,　environmental　effect　was　significant.　Therefore　it　is　important　to　remove　all　kinds　of　interference　in　order　to　obtain　accurate　measurement　results.