The invention belongs to the technical field of nano materials and particularly relates to a preparation method of nano-TiO2. The preparation method comprises steps as follows: step 1, N-butyl titanate is added to absolute ethanol and ultrasonically dispersed for 20-40 min, and a Ti alcohol solution is obtained; step 2, polyvinylpyrrolidone is added to the Ti alcohol solution and stirred until polyvinylpyrrolidone is completely dissolved, then sealed ultrasonic reaction is performed for 1-3 h, and a Ti alcohol dispersion is obtained; step 3, methylcellulose is added to water and stirred at constant temperature until methylcellulose is completely dissolved, and a dispersed aqueous solution is obtained; step 4, the dispersed aqueous solution is slowly dropwise added to the Ti alcohol dispersion and subjected to the ultrasonic reaction for 2-4 h, and a mixed slightly-turbid solution is obtained; step 5, the mixed slightly-turbid solution is added to a reaction kettle and stirred at the constant temperature to be gelatinized, a product is filtered while still hot and washed by absolute ethanol at the constant temperature, and gel is obtained; step 6, the gel is placed in aqueous ethanol, subjected to the ultrasonic reaction for 1-3 h and filtered, then ultraviolet light activation is performed, and nano-TiO2 is obtained.