Nitrogen and sulfur doped titanium dioxide photocatalysts were prepared by the sol-gel method. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-visible diffuse reflectance spectra (DRS). Photocatalytic activities of the samples were investigated on the degradation of methyl orange (MO). The effect of the dopants on the electronic structure of TiO₂ was studied by the first-principles calculations based on the density functional theory (DFT). The orbital hybridization resulted in energy gap narrowing and electronic delocalization in the crystal of doped TiO₂. Mobile electrons of varied energetic states could offer enhanced electron transfer, together with optical absorption improvement. The results show that the doping elements of N and S play a cooperative role in the modification of electronic structure, which enhances the photocatalytic performance. The experimentally observed absorption edges of N-doped TiO₂, S-doped TiO₂, and N, S-codoped TiO₂ are 420, 413, and 429 nm, respectively, which can be explained by the theoretical calculation results.