The structures of (001), (110) and (100) crystal faces, as well as the structure of formaldehyde absorbed on the (001) face of anatase by six kinds of modes were optimized with density function theory (DFT). It is found that the energy of (001) crystal face is the lowest, and that the mode of formaldehyde absorbed on the (001) face of anatase beside H—C—O is the most stable, and has the biggest absorption energy, and taking place chemical absorption. When this absorption was conducted, the C—H bond length increases, and bond strength weakens, while the C—O bond length decreases, and bond strength increases. Meantime, the overlaps of the electronic clouds between titanium and oxygen atom in neighboring layer, and between titanium and carbon atom in formaldehyde, as well as new Ti—O and Ti—C bonds were found. The energy gap between the before and after adsorptions on anatase TiO₂ (001) is decreased from 2.88 to 2.50 eV, the wave length of adsorption light is increased from 431 to 496 nm.