By　establishing　a　three-dimensional　(3D)　numerical　simulation　of　the　Ti-6Al-4V　Gas　Metal　Argon　Welding　(GMAW)　molten　pool,　the　molten　pool＇s　heat　transfer　and　fluid　flow　behavior　under　a　longitudinal　magnetic　field　were　investigated.　The　simulation　results　show　that　when　the　droplet　enters　the　molten　pool,　the　liquid　metals　on　the　molten　pool＇s　surface　symmetrically　flow　towards　both　sides　of　the　molten　pool　from　different　angles.　With　the　increase　of　the　magnetic　field　strengthens,　the　temperature　gradually　decreases,　and　the　fluid　flow　velocity　increases　continuously.　Besides,　the　magnetic　field　strength　is　correlated　positively　with　the　molten　pool＇s　size　with　a　certain　range　of　0-0.03　T.　However,　when　the　magnetic　field　strengthens　reach　0.04　T,　the　magnetic　field　is　correlated　negatively　with　the　molten　pool＇s　size.　Because　the　Marangoni　and　buoyancy　begin　to　weaken,　the　molten　pool＇s　length　change　occurs　before　the　width　change.　Simultaneously,　a　sizeable　velocity　region　appears　on　the　left　side　of　the　molten　pool.　Thus,　the　liquid　metal　gathers　on　the　left　side,　resulting　in　the　weld　cross-section＇s　asymmetry.　It　can　conclude　that　only　when　the　magnetic　strengthen　keeps　in　the　range　of　0-0.03　T,　the　longitudinal　magnetic　field　can　make　the　molten　pool＇s　surface　profile　smooth.