The invention discloses a mould for forming a prefabricated exhaust pipeline. The mould comprises an outer template and an inner template. The outer template comprises an outer bottom template and two outer lateral templates matched with the outer bottom template. An outer mould cavity is formed by encircling the outer bottom template and the two outer lateral templates. The inner template is encircled to form a core mould in the outer mould cavity. A forming cavity is formed between the core mould and the outer mould cavity. An end frame is arranged between the outer wall of the core mould at the two ends of the forming cavity and the inner wall of the outer mould cavity. Inner mould support frames are arranged at the two ends of the inner cavity of the core mould. An outer limiting device is arranged on the outer wall of the outer mould cavity. According to the mould disclosed by the invention, the production of a mechanical mould of a thin-wall exhaust pipeline can be realized; the problems of incapability of mechanical production of a prefabricated non-circular thin-wall component in the forming field and the like can be solved; products are formed at one time; the production efficiency is increased; the labour intensity of workers is reduced; the product intensity is high; the quality is good; simultaneously, the quality fluctuation and the raw material waste resulted from manual operation can be reduced.