Abstract: Deep penetration K-TIG welding is an efficient welding technology widely used in the manufacturing of medium and thick plates. To investigate the mechanism of magnetic field affecting the arc shape, digital image processing technology was used to study the arc shape under different magnetic field strengths. The results showed that the axial magnetic field had different effects on the upper and lower parts of the arc, with the upper part exhibiting greater stiffness than the lower part. Under the same welding parameters, the fluctuation amplitude of the upper arc affected by the magnetic field was smaller than that of the lower arc, and the effect of magnetic field strength on the arc exhibited a nonlinear relationship. The K-TIG welding arc belongs to a free arc, and the formed molten pool area is relatively large. In the transverse welding process, the molten pool flows downward due to the effect of gravity, which affects the formation of the weld. To solve this problem, a method of using magnetic field to assist in suppressing the downward flow of the molten pool was proposed. By the interaction between the axial magnetic field and the current of the molten pool, an electromagnetic force opposite to the direction of gravity was generated to suppress the downward flow of the molten pool. Therefore, the axial magnetic field is suitable for improving the formation of K-TIG transverse welds.