A density functional theory method has been employed to investigate the crystal structures, band structures and photoelectric properties of oxygen-deficient rutile TiO_2-x(x=0~0.5). Defected states composed of Ti 3d and O 2p appear in the band gap owing to the introduction of oxygen vacancies. These defected states reduce the energy for electrons jumping from valence bands to conduction bands, so the absorption edge of TiO_2-x has a red shift, and the absorption intensity of visible light is enhanced, which is beneficial to the photocatalytic activity under visible light. The band gap first decreases and then increases with the increasing of oxygen vacancy concentration (θ) from 1.39% to 25%. When θ=6.25%, the band gap is the smallest which is 1.46 eV. Moreover, according to the UV-vis absorption spectra, it could be confirmed that θ=6.25% is the best oxygen vacancy concentration to improve the photocatalytic activity of TiO₂ because of the large electron mobility and the highest absorption ability of visible light.