The　electric　arc　is　a　critical　factor　affecting　the　performance　of　high　voltage　DC　(HVDC)　relay.　It　is　necessary　to　study　arc　motion　characteristics　and　arc　extinguishing　methods　to　improve　its　electric　life　and　reliability.　A　three-dimensional　mathematical　model　of　arc　plasma　of　HVDC　relay　was　established　based　on　magnetohydrodynamics　(MHD).　The　simulations　on　the　characteristics　of　arc　motion　were　carried　out　under　different　external　magnetic　fields,　different　concentrations　of　silver　vapor　and　additional　axial　insulated　plates.　The　arc　voltages　under　different　conditions　were　obtained.　The　arc　motion　was　characterized　by　the　position　coordinates　of　arc　roots,　and　the　curve　of　arc　root　motion　was　obtained.　The　arcing　process　was　captured　by　high-speed　camera　to　verify　the　simulation　results.　The　results　show　that　the　silver　vapor　generated　from　the　contacts　hinders　the　arc　movement　and　extinction.　The　main　stage　of　arc　erosion　is　that　the　arc　moves　randomly　in　a　small　range　after　the　contact　is　broken　under　the　effect　of　an　external　magnetic　field.　Increasing　the　applied　magnetic　field　intensity　is　beneficial　for　shortening　the　random　motion　time.　The　addition　of　axial　insulated　plates　will　increase　the　arc　voltage　and　accelerate　the　extinction　of　the　arc.　©　2019,　Electrical　Technology　Press　Co.　Ltd.　All　right　reserved.