There is provided a method for preparing an anatase type titanium dioxide photocatalyst having a particle size of nano level without a need of the sintering process at high temperature, and an anatase type titanium dioxide photocatalyst having a particle size nano level. The method for preparing an anatase type titanium dioxide photocatalyst having a particle size of nano level includes adding a titanium-based starting material to a selected solvent and adding an acid or base catalyst to the resulting aqueous solution. Next, subjecting the catalyst-containing aqueous solution to heat treatment at about 80.+-.20.degree. C. to activate peptization thereby preparing an anatase type titanium dioxide sol solution. Finally, the anatase type titanium dioxide sol solution is coated onto a support to complete the preparation of the photocatalyst.