Abstract: The cause and rule of weld formation of mild steel,stainless steel and LD 10CS aluminum alloy are studied in GTAW (tungsten inert gas arc) welding with a longitudinal intermittent alternative magnetic field,by measuring the parameter and taking the weld arc photograph.Under the research conditions described in this paper,the actions of GTAW welding arc property,fluid flow in pool and solidification process are changed due to the applied longitudinal intermittent alternative magnetic field,which produced by the single excitation coil above or under the pool,so the weld formation changes.When the excitation coil and arc center are non coaxial,the effect of applied magnetic field on weld formation is stronger,especially for the ferromagnetism metal.