The P-GMAW is carried out on a 5083 aluminum alloy sheet with a thickness of 10 mm. Under the condition of welding current of 240 A,the arc shape,metal transfer and molten pool behavior during P-GMAW process with or without external longitudinal magnetic field are compared. The results show that the external magnetic field causes the arc to rotate rapidly,the arc length becomes shorter,the droplet is rotated by the tangential force,the neck is obviously constricted during the droplet transfer,and the gray floating object appears on the surface of the molten pool. The corrugation on both sides of the molten pool is uneven,and the contour of molten pool is shifted to one side;the external longitudinal magnetic field does not change the symmetry of arc shape and metal transition,but the molten pool is subjected to an external electromagnetic force,which affects the symmetry of the molten pool flow,thereby the weld offset appears.