The invention relates to the technical field of spinning, in particular to a method for forming composite yarn. According to the method, a production device for forming the composite yarn is used, andthe production device comprises multiple spinnerets, a heating device, a first hot roller, a second roller and the like, the heating device can perform nonuniform heating on tows, the heating devicecomprises multiple guide wheels, a first track, a second track, multiple heating pipes located on the tracks, heating pipe fixing blocks for fixing the heating pipes and the like. The heating device is convenient to assemble and produce and low in cost. The production device adopts the heating device, and compared with the a traditional composite yarn spinning production device, the device has theadvantages that the production cost is low, the slub size and spacing distance of slub yarn can be adjusted, and meanwhile the colorful composite yarn can be produced.