Abstract: To address the difficulty of extracting welding seam information under the action of a longitudinal magnetic field, a “mountain”-distributed longitudinal magnetic field sensor composed of three longitudinally distributed magnetic induction coils was designed. The arc shape under the action of an asymmetric longitudinal magnetic field was simulated using COMSOL software. The magnetic induction intensity corresponding to the arc voltage distribution during welding was considered the magnetic induction intensity for weld identification experimentation. The arc trajectory under the action of the asymmetric longitudinal magnetic field was photographed with a high-speed camera and compared with the arc trajectory designed by the new sensor to verify changes in the arc shape of the asymmetric longitudinal magnetic field generated by the longitudinal magnetic field sensor. The results show that the asymmetric longitudinal magnetic field can control the arc to identify the weld seam and can solve undercut and sidewall non-fusion in the narrow gap welding process. This method opens a new direction for applying magnetron seam tracking sensors in narrow gap welding.