The invention discloses a surface sizing agent added with nanoparticles and a preparation method thereof, the surface sizing agent comprises plant starch, cellulose, cationic styrene-acrylic emulsion,sodium carboxymethyl cellulose, polyvinyl alcohol, modified nanoparticles, a layered silicate and a defoamer; and the modified nanoparticles include nano titanium dioxide, nano silica, a silane coupling agent, methyl methacrylate and potassium persulfate. The preparation method comprises the steps of: mixing the nano titanium dioxide and the nano silica, grinding with the silane coupling agent, and then grafting the obtained surface-modified mixed particles with the methyl methacrylate water for modifying, and finally adding a colloidal solution of the layered silicate and the modified nanoparticles to starch paste to obtain the surface sizing agent. The surface sizing agent is modified by the nanoparticles and is added with bentonite, so that the prepared surface sizing agent has high strength and good water resistance.