The invention relates to a preparation method of polyamide-imide copolymer fiber, which belongs to the technical field of spinning fiber, comprising (1) dissolving trimellitic anhydride and dianhydride monomers in an aprotic polar solvent, under nitrogen protection, Stir at low temperature to obtain a mixed solution; add diphenylmethane diisocyanate monomer in batches, and gradually heat up to 130-180°C to obtain a copolymerized modified polyamide-imide spinning stock solution; (2) polyamide-imide After vacuum defoaming and filtration, the spinning stock solution passes through the spinneret and enters the primary coagulation bath to obtain primary fibers, and then passes through the secondary coagulation bath, hot water drawing, washing, drying, oiling, and dry heat drawing Stretching, heat setting, cooling and oiling and winding to obtain polyamide-imide copolymer fibers. The preparation method of the invention has simple process, and the intermediate acid is easy to store, and the prepared polyamide-imide copolymer fiber has the characteristics of high strength and excellent ultraviolet aging resistance.