Microstructures and properties of 4 mm AZ31 B magnesium alloy welded joints welding by TIG were studied respe-ctively through tensile tests,bending tests,hardness tests and analysis in this paper. The tensile test results showed that,all the specimens were broken in the heat affected zone and the tensile strength satisfied the standard. The grain size of the TIG weld zone is relatively small,the organization is α-Mg solid solution,the grain boundary is dispersed with the black β-Mg17 Al12 precipitation phase,and the grain size of the heat affected zone is longer than that of the base metal. The hardness of the base metal is higher than that of the heat area and weld,there is a softening phenomenon in the joint heat affected zone. It also can be seen that the cracked position of the pulsating tensile fatigue fracture specimen of the 4 mm AZ31 B magnesium alloy TIG welding joint（removed excess weld metal） is in the heat affected zone.