Abstract: Based on theoretical analysis and experimental method, a set of parallel longitudinal magnetic field generating device is used on the non-consumable gas shielded arc welding process, which includes plasma arc welding and tungsten arc welding. With the use of a high-speed video camera observation the arc shape are measured. Then the arc changes of the two kinds of welding method under the longitudinal magnetic field are compared. The experiment results refer that owing to the specifieness of the composition of the plasma arc, the differences between the two kinds of the welding method under the same magnetic field occur. And by means of making analysis of the movement of the particle, the particle motion model is established under the longitudinal magnetic field. It illustrates that because of the difference of the longitudinal velocity of the particles the difference of the motion stage of the charged particles under the longitudinal magnetic field will appear, which explains that why the difference of the arc shape changes exists between the two non-consumable gas shielded arc welding under the same magnetic field.