Abstract: When the welding speed is beyond a critical value, humping bead will occur in conventional gas metal arc welding (GMAW) process. To prevent the occurrence of humping bead, an external magnetic field is exerted on the weld pool to produce a forward electromagnetic force and to impede the backward flow metal jet. A three-dimensional numerical model is established to calculate the distribution of external magnetic field on work-piece before welding. And a thermal-magnetic coupled analysis is presented to calculate the distribution of magnetic field in weld pool during the welding process. Simulation results show that the applied magnetic field is in transverse direction in the weld pool regions. The distance between the wire and the magnet pole plays an important role in affecting the distribution of the external magnetic field.