A method for the preparation of prepreg of thermoplastic composite resin is disclosed. The method comprises the steps of: laminating a non-oriented glass fibrous mat experiencing needle punching on a base layer of a polypropylene film; forming a complex layer which comprises a plurality of yarns unidirectionally oriented on the both sides of a polypropylene film on the non-oriented glass fibrous mat atop the base layer; overlying a needled, non-oriented glass fibrous mat on the complex layer of the unidirectionally oriented glass fibrous yarn/polypropylene film/unidirectionally oriented glass fibrous yarn; covering a top layer of a polypropylene film over the needled, non-oriented glass fibrous mat, to form a laminate; and pressing the laminate at high temperatures under high pressures. By virtue of employing the unidirectionally oriented glass fibrous yarns which can be satisfactorily impregnated into the polypropylene resin, the prepreg of thermoplastic composite resin is superior in mechanical strength, thermal resistance and fluidity. Thermoplastic composite material containing unidirectionally oriented glass fibers with no damage can be used as substance requiring high strength, such as construction materials, machine elements and etc and as articles requiring compress molding, such as auto parts, especially bumper beam.