Abstract: Tungsten inert gas (TIG) welding of AZ31 magnesium alloy sheets under AC longitudinal magnetic field was conducted.After welding,the forming factor,tensile strength and hardness of the resultant joints under different welding parameters were tested,and the microstructure was examined,to investigate the effects of magnetic field on the forming characteristics, microstructure and mechanical properties of the joints.The results show that the AC longitudinal magnetic field changed the moving form of the welding arc and welding pool to control the heat dissipation and crystallization in the welding pool,increase the forming factor of the weld,refine the weld grains and improve the mechanical properties of the joint.When magnetic field current Im was 2 A,magnetic field frequency f was 20 Hz, the mechanical properties of the welded joint were optimized with tensile strength σ_b equal to 231 MPa,elongation percentage δ 11.5%,ratio of shrinkage φ 14.8%,hardness 76.2 HV and forming factor 4.06.