The invention discloses a preparation method of an antibacterial agent, a thermoplastic resin composite material and a preparation method thereof, and relates to the technical field of high polymer materials. The preparation method of the antibacterial agent comprises the following steps: adding fullerene, potassium nitrate, concentrated sulfuric acid and potassium permanganate into water, and stirring at 30-40 DEG C to obtain a mixed solution; adding a hydrogen peroxide solution into the mixed solution, reacting, and removing potassium permanganate to obtain a reaction solution; filtering the reaction liquid to obtain a solid A, and washing and drying the solid A to obtain fullerene oxide; mixing fullerene oxide, water, hexadecyl dimethyl tertiary amine, ethanol and sodium hydroxide, stirring and reacting for 10-14 hours, filtering to obtain a solid B, and washing and drying the solid B to obtain modified fullerene; and mixing the modified fullerene, sodium octadecyl benzene sulfonate, potassium borohydride, zinc nitrate, N, N-dimethylformamide and water, reacting for 12-16 hours, filtering to obtain a solid C, and washing, drying, grinding and sieving the solid C to obtain the antibacterial agent. The antibacterial agent prepared by the method can obviously improve the antibacterial effect.