Abstract: The forming of thin plate lap joints are more difficult to control than that of other joints, and the addition of magnetic field during the welding process can effectively improve the weld formation.The influence of applied low-frequency pulsed longitudinal magnetic field on TIG lap welding arc and weld formation was studied.A high-speed camera was used to photograph the TIG welding arc under the action of magnetic field, a microscope was used to measure the weld width and penetration, and the deflection angle of the arc axis relative to the torch axis was measured.The results show that under the action of external pulsed longitudinal magnetic field, the arc rotates along the torch axis, and with the pulse frequency increasing, the arc rotates faster; when the magnetic field frequency is 100 Hz, the melting width and penetration reach the maximum, the melting width is 10.87 mm and the penetration is 5.36 mm, and the arc deflection angle is the largest of 41°.