The invention relates to a hemming device for a sewing machine. The hemming device comprises a hollow conical main body, and an output port which is connected on the front end of the conical main body; the aperture of the front end of the conical main body is less than that of the back end of the conical main body; the output port comprises an upper sheet body and a lower sheet body; a channel for cloth to pass through is formed between the upper sheet body and the lower sheet body; the one side of the upper sheet body and one side of the lower sheet body, close to the conical main body, are connected with each other, and one side of the upper sheet body and one side of the lower sheet body, far away from the conical main body, are separated from each other; and a needle hole is respectively formed in the upper and lower sheet bodies. The hemming device provided by the invention is employed to assisting machining; and in addition, the hemming device is simple in structure, low in cost, convenient to operate, and capable of effectively improving the machining efficiency.