A method for preparing a gel containing nanometer titanium dioxide particles for visible light photocatalysis, the method has the following acts of: obtaining titanium hydroxide; inverting titanium hydroxide into titanium dioxide by adding an oxidant, an improving agent, an optional acid, and an optional surfactant to compose a solution; and aging the solution by heating to make the solution become a gel. The gel made by the present invention has photocatalystic characteristic and self-cleaning efficiency particularly in visible light but not in ultraviolet light as conventional gel.