Abstract: The AZ31B magnesium alloy plates of 5 mm in thickness were welded by A-TIG welding under longitudinal magnetic field.The mixed oxides were used as activating fluxes.The form factor,microstructure and mechanical properties of welded joint at different magnetic field parameters were tested.The effects of magnetic field parameters on A-TIG welding process of magnesium alloy were also investigated.The results show that the formability and microstructure of magnesium alloy welded joint can be improved due to the magnetic field.When the magnetic frequency is 10Hz and the magnetic current is 2 A,the form factor and properties of welded joint are tested,the form factor is 2.304 and the hardness is 980.98 MPa.The flow of molten pool will be changed by the activating fluxes and electromagnetic (which forming by magnetic field).The liquid metal in molten pool will move from all around to center,which makes the weld be deeper,refines the microstructure and improves the properties of welded joint.