Abstract: AZ31 magnesium alloy sheet with 7.4 mm thickness were welded by AC-TIG welding under different welding current.The microstructure and mechanical properties of welding joint of AZ31 magnesium alloy were studied by optical microscopy, scanning electron microscopies, energy spectrometer, X-ray diffractometer, tensile test machine and micro-hardness instrument. The results showed that the microstructures of the weld zone were fine equiaxed α-Mg matrix and β-Al12Mg17 phase which precipitated on grain boundaries, and the heat-affected zone was overheated structure, the grains of seam and heat-affected zone coarsened with the increasing welding current. The effect of welding current on mechanical properties of the welding joint was larger, the tensile strength of welding joint began to increase and then decrease with the increasing welding current. With 190 A welding current, the tensile strength of joint was up to 88% of the base material, the fracture with mixed toughness and brittleness occured in the heat-affected zone. The microhardness of the joint showed a W-shaped distribution, the hardness value of weld was higher than that of the base metal, and heat affected zone had a lowest value.