The invention provides a dispersing agent and a preparation method and application thereof in dispersion of nano titanium dioxide. The dispersing agent is prepared in the steps that an amphiphilic block copolymer forms a micelle sample, then an ethyl alcohol solution of a functional cross-linking agent and an aqueous solution of the micelle sample are added into a round-bottom flask in the mass concentration proportion of 1:10, and after magnetic stirring is conducted for 48-72 h at room temperature, a solution of a shell cross-linked nanoparticle sample is obtained; then, freeze drying is conducted on a freeze dryer under the condition that the vacuum degree is 1 Pa and the temperature is -60 DEG C, and the dispersing agent is obtained. Nano titanium dioxide is dispersed in a water system through the high-speed stirring and ultrasonic dispersing technology, nano titanium dioxide particles have positive charges in a neutral water medium, so the dispersing agent has anion groups capable of being adsorbed to the surface of nano titanium dioxide and has a certain size to serve as steric hindrance, an adsorption layer with the thickness of a dozen of nanometers is formed, stable dispersion of nano titanium dioxide is achieved under the combined action, and the average particle size of nano titanium dioxide can reach 70 nm or below.