Abstract: In this paper,the microstructures,phase constituent and micro zone analysis of AZ31B wrought Mg alloy welded joint were investigated by optical microscope,XRD and EPMA.The results showed that the fibrous structure of AZ31B Mg alloy disappeared;the fine crystal grain appeared in weld metal and the microstructure of heat affected zone (HAZ) obviously became coarse.The fine equiaxed grains were observed in the weld metal,the microstructure at room temperature consists of δ-Mg phase and compound of γ-Mg₁₇Al₁₂.It found the Mg content in the weld metal is lower than parent metal,however,Al content is higher than parent metal,at the same time there are the segregation of Mg,Al and Zn elements in weld metal.