The invention relates to an after-finishing flame resistant method for a nylon fabric. The method comprises the following steps of: soaking the pre-treated nylon fabric into acrylic amide solution, and performing photo-grafting reaction of acrylic amide on the surface of the nylon fabric under the irradiation of ultraviolet light after a padding process of conventional finishing. By using the principle that a light initiator can extract active hydrogen from the surface of the nylon fabric under the irradiation of the ultraviolet light to form macromolecular free radicals, the method modifies the surface of the nylon fabric and introduces more amide groups into the side chain of polyamide so as to improve the nitrogen content of the polymer and fulfill the purpose of modifying the flame resistance of the nylon fabric.