The invention discloses an adjustable garment top fly multilayer hemming automatic forming device. The device comprises a base, a zipper conveying groove and two hemming mechanisms, wherein the two hemming mechanisms are hinged to stand columns through a cantilever, and the two hemming mechanisms are arranged back to back. Each hemming mechanism comprises a hemming body, a pressing tongue and a supporting tongue, wherein the hemming body comprises an input end and an output end; the output end is subjected to primary bending along the center line in the input direction; the two sides of the bent end of the hemming body are provided with folded edges which are subjected to inward secondary bending, and the supporting tongue is horizontally arranged between the two folded edges and exposed out of the output end of the hemming body; the width of the exposed end of the supporting tongue is larger than the width of the output end after two times of bending; the root portion of the pressingtongue is arranged at the input end, and the other end of the pressing tongue is a free end. The device has the advantages of being simple in structure and easy to manufacture, can achieve automatic secondary hemming of a fabric and automatic sewing of top fly zippers, and greatly improves the production efficiency.