The preparation method of nanometer titanium dioxide modified acrylic polyurethane topcoat belongs to the field of nanotechnology and corrosion protection technology. The invention solves the problems of the aging resistance of the acrylic polyurethane topcoat in the prior art and the poor resistance to salt spray immersion and the like. The preparation method of the present invention includes: firstly adding a coupling agent to a titanium dioxide solution for high-speed dispersion, and obtaining modified nanometer titanium dioxide powder through steps such as concentration; and then sequentially adding a solvent, a pigment and a filler to an acrylic polymodified polyurethane resin With the catalyst, fully react to obtain the acrylic polyurethane resin; finally, the modified nano titanium dioxide powder is added to the acrylic polyurethane resin for grafting reaction, curing, and drying to obtain the nano titanium dioxide modified acrylic polyurethane topcoat. The topcoat prepared by the invention has a simple preparation method, and has high application value in coastal buildings, steel bridges, petroleum and petrochemical pipelines, storage tanks and other facilities.