Based　on　theoretical　analysis　and　experimental　method,　a　set　of　parallel　longitudinal　magnetic　field　generating　device　is　used　on　the　non-consumable　gas　shielded　arc　welding　process,　which　includes　plasma　arc　welding　and　tungsten　arc　welding.　With　the　use　of　a　high-speed　video　camera　observation　the　arc　shape　are　measured.　Then　the　arc　changes　of　the　two　kinds　of　welding　method　under　the　longitudinal　magnetic　field　are　compared.　The　experiment　results　refer　that　owing　to　the　specifieness　of　the　composition　of　the　plasma　arc,　the　differences　between　the　two　kinds　of　the　welding　method　under　the　same　magnetic　field　occur.　And　by　means　of　making　analysis　of　the　movement　of　the　particle,　the　particle　motion　model　is　established　under　the　longitudinal　magnetic　field.　It　illustrates　that　because　of　the　difference　of　the　longitudinal　velocity　of　the　particles　the　difference　of　the　motion　stage　of　the　charged　particles　under　the　longitudinal　magnetic　field　will　appear,　which　explains　that　why　the　difference　of　the　arc　shape　changes　exists　between　the　two　non-consumable　gas　shielded　arc　welding　under　the　same　magnetic　field.　©,　2014,　Harbin　Research　Institute　of　Welding.　All　right　reserved.