This invention discloses a method for preparing highly active titanium dioxide nanosol. The method comprises: (1) weighing titanium oxysulfate or titanium sulfate, adding water to dissolve, dropping base solution to adjust the pH value to above 8, and stirring adequately; (2) filtering, washing the precipitate until there is no sulfate ions, and repeatedly washing with distilled water to neutrality; (3) adding water to the precipitate, mixing uniformly, beating, transferring the slurry to a constant-temperature reaction kettle, adding water, mixing uniformly, dropping acid solution to adjust the pH value to 0.5-2.0, and stirring adequately at normal temperature; (4) stirring the slurry at 40-95 deg.C for 3-60 h; (5) performing high-pressure hydrothermal treatment. The obtained titanium dioxide nanosol is light blue, and does not have any odor or organic matter. Besides, the titanium dioxide nanosol has such advantages as high stability, high crystallinity, high activity, high light transmittance rate, low electron-cavity recombination, high antibacterial and moldproof ability, and no corrosion to matrix. The method has such advantages as simple process and easy operation.