Composite　insulators　in　valve　tower　of　UHVDC　converter　station　withstand　combined　ac-dc　voltage.　The　flashover　performance　of　silicone　rubber　under　combined　voltage　is　investigated　in　this　paper.　The　voltages　on　the　post　insulator　of　valve　tower　under　different　operating　conditions　are　simulated　in　PSCAD/EMTDC,　and　the　harmonic　components　of　the　voltages　are　analyzed.　The　simulation　results　indicate　that　the　ac　frequencies　of　the　voltage　on　the　post　insulator　are　mainly　50　Hz,　3n　and　6n　+/-　1　times　harmonics.　The　flashover　voltages　of　silicone　rubber　under　different　combinations　of　ac　and　dc　voltages　are　tested,　including　different　single　frequency　ac　voltages　superimposed　on　different　dc　voltages,　and　different　combined　harmonic　voltages　superimposed　on　different　dc　voltages.　The　test　results　indicate　that　different　ratios　of　dc　voltage　to　ac　voltage　and　different　percentages　of　harmonics　have　various　influences　on　the　flashover　voltage　of　silicone　rubber.　Moreover,　argon　atmosphere　pressure　plasma　jet　(Ar　APPJ)　is　applied　to　modify　the　surface　of　the　silicone　rubber　under　different　modification　times　and　voltages.　The　flashover　voltages　and　the　surface　morphologies　of　the　silicone　rubber　before　and　after　surface　modification　are　compared.　Surface　modification　by　Ar　APPJ　can　improve　the　surface　resistivity　of　the　silicone　rubber　significantly,　and　the　flashover　voltage　also　increases.