Abstract: During the welding of AZ31 magnesium alloy plate(5 mm), AC longitudinal and variable frequency magnetic field was used, and the tensile strength and microhardness of welded joint at different magnetic field parameters were tested.With scanning electronic microscope and metallographic microscope, the microstructure of welded joint was analyzed.The effect of magnetic field on the mechanical properties and microstructure of welded joint were studied, and the function mechanism of magnetic field was explored.The results show that the AC longitudinal magnetic field stirs welding pool through the rotating arc, changes the crystallizing process of crystal grains to refine them, which makes the hardness and tensile strength of welded joint improved.Under the proper magnetic field parameter, the electromagnetic stirring achieves the best effect.When magnetic field current I_c=2 A, frequency f=20 Hz, the mechanical properties of welded joint will reach the best value.The hardness is HV 76.2, and the tensile strength is 231 MPa, which is 90% of base metal.