The invention discloses a reinforcing adhesive, a fiber-filled thermoplastic modified plastic, and a preparation method of the plastic. The reinforcing adhesive comprises, by mass, 10-90 parts of a diluent, 10-50 parts of a film-forming agent, 10-50 parts of an adhesive and 0.5-5 parts of a stabilizer. The reinforcing adhesive for the fiber-filled thermoplastic modified plastic of the invention can be used to easily coat or impregnate fibers, has excellent processability, and can firmly bond fibers and the thermoplastic plastic after the diluents is completely removed in order to achieve the purpose of preparing the fiber-filled thermoplastic modified plastic with high mechanical properties.