Abstract: According to the actual production demand of friction stir welding circumferential welding, a special fixture with fixed diameter inner support is designed. The fixture can realize joint expansion between the combined support blocks in the whole operation process, and has the functions of automatic centering and self-locking, which solved the technical problems such as time difference in place, collision impact and poor circularity between positioning elements. The fixture can not only effectively improve the manufacturability and assembly of product welding, but also provides a new method and idea for the fixture design.