Abstract: For the problem of poor forming in the process of microbeam plasma welding caused by the downward flow of molten metal on both sides of thin-wall parts, the analysis of molten pool mechanical behavior and the research of pool forming control strategy are studied. The method of the external magnetic field is proposed for adding the inside of the electromagnetic pressure to the molten pool, intervening the downward flowing molten metal. The finite element software COMSOL is used to simulate the magnetic flux density, the induced current density and the electromagnetic force on the workpiece. Experiments of single-channel multi-layer welding are performed on 304 stainless steel, and the welding cladding morphology under different magnetic flux density was obtained. The results show that the magnetic field can significantly change the mechanical behavior and forming rules on the microbeam plasma welding, and can effectively restrain the flow of molten metal on the edge.