The invention relates to a positioning device of a smoke box joining machine. The positioning device comprises a driving device capable of driving a connecting plate to perform a linear reciprocating motion in a direction parallel to a slide rail; one end of the connecting plate is fixedly connected with an output end of the driving device, and the other end of the connecting plate is fixedly connected with a mounting base; the mounting base is slidingly arranged on the slide rail; the mounting base is provided with a positioning block for positioning a smoke box plate; and the positioning block comprises a first positioning surface and a second positioning surface respectively matched with two adjacent surfaces of the smoke box plate. The positioning device further comprises a positioning plate corresponding to the positioning block; and the positioning plate comprises a third positioning surface parallel to the first positioning surface, and a fourth positioning surface parallel to the second positioning surface. The positioning device of the smoke box joining machine is simple in positioning operation, high in automation degree and suitable for producing smoke boxes with different specifications.