Abstract: The AC TIG was applied on AZ91 magnesium alloy plate(in 5mm thickness).The effect of different welding currents on microstructure and properties of surfacing layer was investigated by X-ray diffraction(XRD),Optical Microscope(OM),Scanning Electron Microscope(SEM),and Tensile Tester.The results show that with the increasing of welding current,the form of welding seam become bad and the grain growth get coarse.At the same time,gas porosity and crack are easy to by produced,which reduce the properties of the joint.The microstructure of the weld zone is composed of α-Mg matrix and β-Al12Mg17 phase.When the welding current is 100A,the mechanical properties of welded joints reach maximum,at this point tensile strength is 252 MPa,percentage elongation is 6.9%.