The experimentation and research of AZ31B magnesium alloy friction stir welding were carried out by using plates of 5 mm in thickness.The results showed that the material suitable for the stir welding head was W6Mo5Cr4V2 high-speed steel.Its structure was in shape of concave round platform,the root of which was 5.5 mm in diameter,the butt 2.5 mm,the shoulder 12 mm,and its length was 4.7 mm.The tensile strength of magnesium alloy friction stir welding joint could be 90% of that of the parent metal,and the specific elongation could be 50% of that of the parent metal.The structure of the weld area was dynamic recrystallization.Apparent boundary existed between the weld area of the forward border joint and the parent metal,while trails of little molten showed in the transitional area of the back border.