The method of co-doping is very useful to improve the photocatalytic performances of titanium dioxide nanotubes. The absorption capacity to the visible light of the titanium dioxide nanotubes can be improved significantly in experiment by doping both N and F in titanium dioxide nanotubes, but the theoretical explanations are still not clear. Doping the atom N alone, the atom F alone, and both N and F in titanium dioxide nanotubes respectively, their atomic structures, electronic properties and optical performance are studied by the first principles method based on the density functional theory. It is found that formation energies are lower in titanium-rich environment than that in oxygen-rich environment. In titanium-rich environment, the N-F co-doped TiO2 nanotube has the low formation energy and stable thermodynamic system compared with the N alone and the F alone doped TiO2 nanotube. Besides, the O3C can be replaced more easily than the O2C when doping N alone, F alone and co-doping N-F in TiO2 nanotube. By analyzing the energy band, we can find that the band gap changes little with doping N and the change of the band gap for the co-doping N-F case is the most prominent, which reduces by 0.557 eV compared with that for the un-doped TiO2 nanotube case, and this is mainly from the contributions of the impurity level near the top of the valence band. Besides, the different charges are calculated and it is indicated that the ability to gain electrons of N is stronger than that of F, and through analyzing the photocatalytic performance, it is found that though the gap of the nanotube is larger than that of the body, the reducibility of nanotube is better than that of the body. Both the reducibility and the oxidability of the nanotube are reduced but its activity is not lost when co-coping the atoms of N and F in titanium dioxide nanotubes. Moreover, the optical absorption spectrum shows that the red shift phenomenon is obvious for doped system and also for the co-doped system. Therefore, co-doping both N and F in titanium dioxide nanotubes is the most useful method to improve the photocatalytic performances of the TiO2 nanotubes.