The invention relates to a preparation method of a composite antibacterial finishing agent. The preparation method comprises the following steps: modifying nano zinc oxide by a silane coupling agent containing double bonds to obtain modified active nano zinc oxide; providing reaction raw materials; taking water as the solvent, and carrying out polymerization reactions among active nano zinc oxide, dimethyl diallyl ammonium chloride, carboxyl acrylic acid monomers, and allyl glycidyl ether in the presence of an initiator to obtain the composite antibacterial finishing agent. The composite antibacterial finishing agent prepared by the provided preparation method is a complex of quaternary ammonium salt copolymer and nano zinc oxide. Through the synergistic effect between quaternary ammonium cations and nano zinc oxide, the antibacterial properties of inorganic materials and organic materials can be fully exerted, the finishing agent can enable textiles to have a very good antibacterial performance, at the same time, the antibacterial finishing agent contains active groups that can interact with cotton fibers, and the antibacterial effect can last for a long term.