Oil-paper　insulation　is　the　most　widely　used　insulation　material　in　converter　transformers,　and　its　safe　and　reliable　operation　is　one　of　the　important　factors　to　ensure　the　normal　operation　of　converter　transformers.　In　the　long-term　working　process,　the　formation　and　accumulation　of　space　charge　in　the　oil-paper　insulation　will　affect　the　oil-paper　interfacial　characteristics　and　the　internal　electric　field　distribution,　and　will　affect　the　breakdown　characteristics.　Therefore,　it　is　of　practical　significance　to　conduct　in-depth　study　on　the　charge　transport　characteristics　and　breakdown　characteristics　and　mechanism　of　oil-paper　insulation　under　the　influence　of　space　charge.　Based　on　the　study　of　oil-paper　insulation　breakdown　characteristics,　this　paper　systematically　studies　the　space　charge　characteristics,　oil-paper　interface　characteristics　of　oil-paper　insulation　and　its　breakdown　characteristics　in　different　states.　Through　this　research,　according　to　the　breakdown　strength　of　oil-paper　insulation　under　different　conditions,　it　can　provide　support　for　the　design　of　UHV　converter　transformers,　provide　experimental　basis　for　improving　the　breakdown　field　strength　of　oil-paper　insulation,　and　provide　a　deeper　understanding　of　the　solid-liquid　interface　characteristics　based　on　theoretical　study.　©　2020　Institute　of　Electrical　Engineers　of　Japan.　All　rights　reserved.