Combining the process requirements of the world’s largest 10 m large-diameter carrier rocket tank bottom friction stir welding, a gantry type welding equipment for the large-diameter spherical body friction stir welding was designed. Due to friction stir welding, high-speed rotating welding head and the welded material are rubbed to generate heat and locally plasticized, when the welding tool moves forward along the welding interface, the plasticized material is flowed from the front of the welding tool to the rear under the action of the frictional force of the welding head, and a dense solid-phase weld is formed under the extrusion of the welding head. Unlike fusion welding, large and very stable upsetting force and stirring force are required in the friction stir welding process. Therefore, the analysis of the mechanical characteristics of the overall structure of the gantry milling and welding equipment is very important. Combining the force required by the 28 mm aluminum alloy material 2219 in the friction stir welding process and the spherical structure of the bottom of the rocket tank, the welding process mechanics equation of the welding joint was established, the static mechanical properties of the gantry milling welding equipment during the welding process were analyzed. The results show that the maximum stress and the maximum deformation of the U-shaped head of the designed gantry milling and welding equipment have a linear relationship with the angle between the stirring head and the bottom of the box during the welding head penetration stage, stable welding stage. In terms of strength, the current designed gantry-type equipment has the disadvantage of excessive margin and can be improved.