Abstract: The mechanism of TIG weld formation under external electromagnetic fields was analyzed theoretically. The external magnetic fields could change the difference of surface tension of liquid metal and then change the flow direction of the molten pool. When ∂σ/∂T > 0,the liquid metal around the edge of molten welding pool flew inward and undercut could occur. However,when ∂σ/∂T < 0,the liquid metal flew outward and the trend of forming undercut decreased. The effects of transverse DC magnetic field,transverse AC magnetic field and longitudinal magnetic fields on the time for welding arc penetrating the weld section were compared. The theoretical analysis results show that the effective diameter of the anode spot of TIG welding arc with external magnetic field was larger than that without external magnetic field. TIG welding with external magnetic field could help to reduce the surface tension temperature coefficient and avoid the undercut and bump seam during high speed TIG welding.