The invention relates to the technical field of powder dispersing, and discloses a preparation method of monodisperse titanium dioxide size. The method comprises the following steps: S1, preparing size through titanium dioxide; then adding a dispersing agent; and uniformly mixing to obtain dispersed materials; S2, mixing impact beads and the dispersed materials; and S3, arranging a vessel in whichthe impact heads and the dispersed materials are arranged on a liquid phase shearing-impact dispersing machine for grinding to obtain monodisperse titanium dioxide size. With the adoption of the method, the dispersing performance and the dispersing stability of titanium dioxide particles can be effectively improved; and meanwhile, the surface of each particle can be covered with a complete film layer in the following titanium dioxide film covering modification process. The method is low in cost, simple to operate, and high in implementing performance; and the application performance of titanium dioxide can be effectively improved.