Abstract: For AZ31B magnesium alloy sheet of 2 mm thickness,spot welding was carried at a three-phase secondary rectification welding machine DZ-3×100.Fracture feature of spot welding joint was studied by the tensile shearing test,optical microscope,SEM,X-ray method.The result showed that fracture of spot welding joint showed two kinds of fracture morphology under tensile-shear loading,one is tearing fracture,the another is whole nugget fracture.The fracture surface was a ductile-brittle mixed mode in the nugget region,whereas in base metal region it was a typical ductile mode.The phase of base metal is basically same as nugget in the spot welding joint.However the hard and brittle Mg₁₇Al₁₂ intermetallic compound is precipitated from intracrystalline or crystalline grain boundary in base metal after being welded,where the proportion is far more than that of base metal.It shows that fracture tendency of nugget region is more serious than base metal,and the diffraction peak of Mg₁₇Al₁₂ is more intense in the XRD patterns.