Abstract: In order to solve the problem of poor bottom forming of magnesium alloy friction stir welding joints, the bottom auxiliary heat friction stir welding of 4 mm thick AZ31B magnesium alloy plate was carried out, and the effect of the bottom auxiliary heat on the temperature of the bottom zone, and the microstrcture and performance of the magnesium alloy joint was studied. The results show that when the bottom auxiliary heat is applied, the heat dissipation at the bottom of the weld is reduced, and the peak temperature is increased. At the same time, the bottom auxiliary heat can soften the metal, improve the fluidity of the material, the hole defects at the bottom of the weld disappear, the bottom structure is refined, and the tensile strength and elongation of the joint are improved.