The　structures　of　high-voltage　direct　current　(HVDC)　relays　are　compact.　In　order　to　extinguish　the　arc,　the　external　magnetic　field　is　applied　near　the　contact.　HVDC　relays　with　a　large　capacity　are　also　filled　with　gas　medium　to　enhance　the　ability　of　arc　extinguishing.　The　research　on　the　characteristics　of　the　arc　in　different　gases　under　different　pressures　is　of　great　significance　to　improve　the　reliability　and　the　lifetime　of　the　HVDC　relays.　The　characteristics　of　the　arc　in　nitrogen,　hydrogen,　and　nitrogen-hydrogen　mixtures　and　that　of　the　arc　root　motion　in　the　magnetic　field　are　simulated　by　means　of　magnetohydrodynamics　(MHD).　The　arcing　processes　in　N-2　and　air　are　tested　by　a　high-speed　camera,　and　the　arc　root　coordinates　are　calculated　by　the　image　processing　method,　and　the　simulation　models　are　verified.　The　result　shows　that　under　the　same　condition,　the　hydrogen　arc　voltage　is　the　highest,　and　the　moving　speed　of　the　arc　root　is　the　fastest,　so　that　the　arc　extinguishing　performance　of　the　hydrogen　is　the　best.　Nitrogen-hydrogen　mixture　is　also　a　superior　arc　extinguishing　gas,　and　the　higher　the　proportions　of　H-2,　the　better　the　performance.　With　the　increase　of　the　pressure,　arc　voltage　increases.　Except　H-2,　increasing　the　pressure　of　the　gas　is　beneficial　for　arc　motion.