Abstract: Welding joints with partial thickened structures are usually adopted in the design of aluminum alloy profiles to offset the thinning of the joint caused by the downward pressure of the shoulder in friction stir welding. However, after welding of such aluminum profiles, it will take a lot of hours to manually polish the thickened structures. In order to reduce the amount of grinding, a new type of aluminum profile without thickened structure is designed in this paper, ensuring non-thinning-welding by changing the structure of shoulder. In-depth research is carried out from the perspective of weld forming quality, mechanical properties and organizational structure, and key process parameters such as welding downforce are determined. Finally, combined with the characteristics of extrusion tolerance and assembly tolerance in industrial production, the influence of joint misalignment on welding quality is studied, and a reasonable joint tolerance range is determined.