Abstract: Ultra-thin-walled stainless steel round pipe is widely used in aerospace, automotive, chemical and nuclear energy fields, because of its light weight, high strength, corrosion resistance and other excellent characteristics, become an important material for high-precision welding requirements. Due to its thin wall thickness and strong thermal sensitivity, the traditional welding process is prone to problems such as excessive heat affected zone, serious welding deformation and unstable performance of welded joints. Therefore, using numerical simulation method to study the process of magnetic-controlled microbeam plasma arc welding of ultra-thin-walled stainless steel round tube can provide theoretical guidance for optimizing welding process and improving welding quality. By numerical simulation of micro-beam plasma arc welding process under alternating transverse magnetic field, the influence of different process parameters on welding process is analyzed, and optimization suggestions are put forward.