The　field　intensity　will　be　strengthened　with　the　electric　field　induced　by　space　charges　added　on　the　applied　electric　field　if　the　voltage　polarity　of　DC　cable　is　reversed,　which　is　possible　to　cause　partial　discharge　or　dielectric　breakdown.　The　transient　variations　of　space　charges　and　electric　field　during　the　process　of　polarity　reversal　is　difficult　to　be　revealed　by　the　present　measurement　methods,　especially　the　interaction　processes　between　microscopic　particles.　In　order　to　deeply　study　the　effects　of　the　polarity　reversal　on　the　insulating　properties　of　HVDC　cables,　the　dynamic　characteristics　of　space　charges　in　low　density　polyethylene　(LDPE)　are　numerically　investigated　by　the　bipolar　charge　transport　model.　Meanwhile,　the　transient　distributions　of　space　charge　and　electric　field　in　LDPE　under　several　polarity　reversals　are　investigated.　The　results　show　that　the　surface　electric　field　is　strongly　strengthened　under　polarity　reversal.　The　field　variation　is　of　the　polarity　effect,　which　is　more　significant　under　the　anode　transiting　to　the　cathode.　The　density　of　trapped　charges　is　much　higher　than　that　of　mobile　charges　before　polarity　reversal.　And　the　strengthened　electric　fields　on　the　electrode　surfaces　under　polarity　reversal　are　dominated　by　the　trapped　charges　in　the　vicinity　of　the　electrodes.　The　results　can　provide　a　reference　for　the　insulation　design　of　high　voltage　DC　cables,　as　well　as　the　dynamic　simulation　of　space　charges　in　polymeric　materials.　©　2017,　High　Voltage　Engineering　Editorial　Department　of　CEPRI.　All　right　reserved.