Abstract: AC lognitudinal magnetic field was employed in welding of 5 mm thick AZ31 magnesium alloy platewith GTAW.The effect of magnetic field parameters on properties and microstructure of welded joint of AZ31 was studied throungh analysing the tensile strength, hardness and microstruture.The action mechanism of magnetic field was explored.The results indicated that the molten pool is stirred by the AC lognitudinal magnetic field, the solidification process is changed, the cry stal grain is refined, and tensile strength and hardness of welded joint are improved.The molten pool is purified by the eletromagnetic stirring, so the gas pore and sensitivity of hot crack are decreased, and the formation of hot crack was supressed.