Abstract: The keyhole variable polarity plasma welding was used to weld AZ31BMg alloy, and favorable joint was obtained.The microstructure, mechanical properties and hardness were investigated by the optical microscope, tensile testing machine and hardness instrument.The results show that one side welding with back formation can be obtained through this technique when there is no groove and no enclosed protection;the heat-affected zone is narrow, and the average tensile shear is 7.3 kN;the grains of fusion zone are fine, and the hardness of welded joint is higher than that of the base metal.In summary, the variable polarity plasma arc welding process is a perfect method to weld Mg alloy, which can improve the microstructure of the joint and increase the welding quality.