In this paper, based the processing method of cross beam used in steel structure factory building as the research object, because the cross beam without special profiles, so we need two profiles spell-welding. The ordinary submerged arc welding equipment cannot normal applied welding, and manual spell-welding cannot guarantee the quality, due to structural constraints. In order to ensure accuracy of cross beam welding, before the design of special welding machine, the error calculation was conducted in view of positioning method of special welding machine, and find out the reasonable positioning mode, provide theoretical support for manufacturing special welding machine.