Abstract: In order to improve the arc energy density of K-TIG welding and weld quality, the effect of longitudinal magnetic field on the arc shape and weld formation was studied. The results show that: under the action of DC longitudinal magnetic field, the bigger the magnetic field intensity, the more obvious the welding arc is compressed. Because the molten pool is affected by Lorentz force, the molten metal shifts to one side of the weld, and the bigger the magnetic field strength is, the more obvious the phenomenon is. Under the action of AC longitudinal magnetic field with constant magnetic field frequency, the bigger the magnetic field intensity is, the more obvious the arc compression effect is; under constant magnetic field intensity, the higher the magnetic field frequency is, the more stable the arc is. Due to the influence of Lorentz force on the molten pool, the molten pool is stirred, thus affecting the weld formation. Whether under the action of DC longitudinal magnetic field or AC longitudinal magnetic field, the arc shape presents a more "thin" bell shape. With the increase of magnetic field intensity or magnetic field frequency, the welding current required for complete penetration of the base metal decreases and the weld width decreases.