Abstract: The high-speed photography system was used to collect the images of MIG welding arc and jet transition under alternating intermittent magnetic field.Through force analysis, the changes of arc and jet transition under different magnetic field directions were studied.The result shows that under the excitation frequency of 40 Hz and the excitation current range of 0-1.0 A, as the alternating magnetic field periodically changes from downward to upward in longitudinal direction, the arc rotates clockwise and counterclockwise periodically around the wire axis, and it expands and contracts.The conductive fluid at the end of the wire consists of a liquid cone, a liquid stream and a small spherical droplet.When the excitation current is 0 A, the liquid cone and the liquid stream sag along the axis of the welding wire without being deflected, and the droplet transits smoothly into the molten pool along the end of the liquid stream.When the excitation current is 0.2, 0.6 A, the liquid stream is short and thick, and it deviates from the axis of the welding wire with a small angle and rotates clockwise and counterclockwise.When the excitation current reaches 0.8, 1.0 A, the liquid stream is slender, and the liquid stream shows an"S"shape, deviating from the axis of the welding wire with a large angle and rotating periodically counterclockwise and clockwise.