Abstract: In order to improve the weld forming and the performance of welded parts, the coupling analysis between electromagnetic field and thermal flow field of 304 stainless steel weld pool under external magnetic field is carried out by finite element method in this paper, and the velocity vector distribution of liquid metal flow in weld pool under magnetic field and without magnetic field is obtained. The results show that the maximum velocity distribution in the cross section of the molten pool is changed from a single central part to a slightly lower upper surface and the bottom of the molten pool, and the maximum velocity distribution on longitudinal section is not only located at the vortex junction of the front molten pool and the tail molten pool but along two vortex flow direction more uniformly distributed. This is because the electromagnetic pressure offset part of the effect of surface tension, and the electromagnetic pressure makes the acting position of the surface tension is more close to the center of the molten pool and away from the edge of workpiece. The building-up welding test on 304 stainless steel is carried out, and the phase diagram of cross section of the welding confirms the above simulation results.