The lattice parameters and band-gap of native rutile TiO2 are investigated by the first-principles calculations of local density approximation+U method with different U values for Ti-3d (0 U 9 eV). The electronic structures and optical properties of different content C doped rutile TiO2 systems are also studied by the same method with appropriate U values. The calculations results show that the lattice parameters and band-gaps of TiO2 increase with the increase of U and the U =3 eV is fitted for the corrected band-gap. For the doped systems, the impurity energy level is introduced due to the coupling between O-2p and C-2p, which can increase the TiO2 absorption edge to the visible region, and therefore enlarge the absorption region of TiO2. Moreover, the 8.3% C is an optimal doped density, which can lead to the red-shift of optical absorption edge obviously and increase the coefficient of light absorption, therefore facilitate the enhancement of the photocatalytic efficiency.