Abstract: A simulation method based on simplified arc model is developed for the analysis of GMAW deposition with various external magnetic fields. The electromagnetic force distributions in the workpiece produced by longitudinal and transversal magnetic fields are systematically investigated in comparison with the case without external field. The results reveal that the inherent electromagnetic force of GMAW welding drive the converging and sinking flow in the central of melt pool. The external longitudinal field results in circumferential electromagnetic force which stirs the melt pool, which in turn refine the microstructure. The external transversal field of specific direction could convert the melt convection into "single-vortex" state which help the melt pool to absorb molten droplet and therefore reduce the spatter. The agreement between experimental data and the simulation results verifies the reliability and universality of the numerical model.