Titanium dioxide (TiO2), as a kind of excellent photocatalyst, has been widely researched and applied. The geometry structures, band structures, densities of states and absorption coefficients of three possible (Nb, N) co-doped anatase are studied by the density functional theory based on GGA+U method, and the results are compared with those of the single doped (Nb/N) anatase. It is shown that the introduction of dopants leads to the lattice distortion. Besides, compared with undoped TiO2, (Nb, N) co-doped TiO2 has small band gap and absorption edge, and the level of N 2p state changes from partially occupied to occupied, which reduces the recombination of electron-hole pairs. In addition, the smaller formation energy of co-doped anatase indicates that it is more stable than the N doped anatase. Therefore, (Nb, N) co-doping anatase is a kind of stable photocatalyst and it has better photocatalytic performance in visible light band than pure TiO2.