Abstract: During the welding of AZ31 magnesium alloy plate(5 mm) by GTAW,AC longitudinal magnetic field was used.The fatigue properties of samples were researched under different magnetic field parameters.Scanning electronic microscope and metallographic microscope were used to analyze the fracture and microstructure of the fatigue samples.The effects of magnetic current on the fatigue properties and microstructure of the 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 grain to refine the crystal grain and make the fatigue properties of welded joint improve.Under the suitable magnetic field parameter,the electromagnetic stirring achieves the best effect.The optimal value of fatigue property appears when magnetic field current I_m=2 A.