Abstract: The longitudinal magnetic field is applied to streaming transfer MAG welding process with the shielded gas of Ar + 2% O₂.With the help of high speed camera,the effect on arc shape and moving behavior is studied.Moreover,the influence on bead formation of exciting current is investigated.The mechanism of the influence of the congitudinal magnetic field on bead formation is disclosed from two angles of the heating character of a rotating arc and electromagnetic stirring.