Abstract: In conventional friction stir welding, the length of the stirring pin needs to be strictly matched with the thickness of the welded plate. A too-long or too-short stirring pin will adversely affect the welding effect. To solve this limitation, this paper proposes a new welding process that adds a backing plate of the same material with appropriate thickness to the back of the weld seam. In this process, the length of the stirring pin is greater than the thickness of the welded plate, and the backing plate is fused with the base metal. On the one hand, the requirement for stirring pin length is reduced. On the other hand, the adverse effects of weld seam thinning can be eliminated. The butt welding of 1.5 mm thick AZ31B magnesium alloy was studied by this method. The maximum tensile strength of the weld joint can reach 91.19% of the base metal. In addition, the microstructure and microhardness distribution of the weld beam cross-section were analyzed. The optimal process parameters were obtained by the established convolutional neural network model, and the distribution of joint tensile strength with the change of parameters was predicted.