The effects of Mn and N codoping on the crystal structure, defect formation energy, electronic structure, optical property and redox ability of anatase TiO2 are investigated by first-principles calculations of plane-wave ultrasoft pseudopotential. The calculation results show that the octahedral dipole moment of anatase TiO2 increases due to its lattice distortion after Mn, N codoping, which is favorable for effective separation of photogenerated electron-hole pairs. Some impurity bands appear in the band gap, which leads to the red-shift of optical absorption edge and to the increase in coefficient of light absorption, thereby facilitating the enhancement of the photocatalytic efficiency. If the impurity band is not taken into account, the band edge redox potential of codoped TiO2 is only slightly changed compared with that of pure TiO2 . All of these results can explain the better photocatalytic performances of Mn, N codoped anatase TiO2 under visible-light irradiation.