The invention discloses a preparation method of nano titanium dioxide with a high catalytic activity. The preparation method comprises the following steps: preparing raw materials: a butyl titanate precursor and diformyl nanocrystalline cellulose, which is obtained through an oxidization treatment; optionally adding a thickening agent and a surfactant, preparing a butyl titanate solution and a diformyl nanocrystalline cellulose solution, mixing two solutions under high speed sheering to obtain a uniform mixture, and subjecting the mixture to a gelation and high temperature calcination treatment to obtain the target product. According to the preparation method, oxidized nanocrystalline cellulose rich in diformyl groups is taken as the basic material, and under the effects of auxiliary materials such as thickening agent, surfactant, and the like, during the process of hydrothermal-freeze drying and gradient-heating calcination, the n-butyl titanate precursor is induced to carry out crystallization so as to form wheat-ear-shaped nano titanium dioxide crystal particles. The crystal particles can be well dispersed in water, thus the efficiency on processing the pigments in wastewater is effectively improved, and the provided titanium dioxide can degrade more than 98.5% of methyl orange within 12 minutes, in the best case.