Highly　damped　oil-paper　insulation　system　is　respected　as　the　predominant　factor　of　surface　discharge　phenomena　in　converter　transformers,　which　consist　AC　and　DC　combined　electric　field.　The　present　study　focuses　on　the　surface　discharge　characteristics　of　oil-paper　insulation　system　with　different　moisture　content　under　AC-DC　combined　voltages.　AC　and　DC　voltages　were　applied　to　the　sphere-plane　defect　model　in　the　established　experimental　platform,　and　various　oil-immersed　pressboards　with　different　moisture　content　were　prepared　according　to　the　cases　in　actual　converter　transformers.　The　developing　processes　from　the　initiation　of　surface　discharge　until　final　flashover　were　observed　for　each　moisture　content　and　DC　component.　PD　parameters　and　patterns　were　acquired　by　detector　using　pulse　current　method.　Results　show　that　moisture　has　a　significant　impact　on　surface　discharge　activities　in　oil-paper　insulation.　It　is　shown　that　the　inception　and　flashover　voltage　decrease　with　the　increase　of　moisture　content,　while　the　amount　and　amplitude　of　discharge　increase.　As　the　damped　state　in　oil-impregnated　paper　samples　gets　more　serious,　the　carbon　trace　on　the　surface　of　pressboard　is　much　more　obvious.　The　process　of　the　whole　surface　discharge　under　AC-DC　combined　voltage　can　be　divided　into　three　stages.　The　findings　of　the　current　study　are　useful　in　further　research　on　fault　diagnose　in　converter　transformers.　©　2018　IEEE.