A method for preparing for a photocatalyst. The method comprises steps of providing a mixture of indium oxide and vanadium oxide and then calcining the mixture to obtain a indium vanadium quadrioxide. Further, a nickel nitrate solution is added to the indium vanadium quadrioxide to form a catalyst with a nickel oxide supported amount of about 0.1-2.0 wt. % and a post treatment is performed on the catalyst. In the post treatment, a reduction process is performed and then an oxidation process is performed.