[en] Bond relaxation and the associated electronic energies mediate the properties of a substance, and therefore, doping could be the effective means for such purpose. With the aid of first principle calculations and experimental observations, we examine four co-doped specimens (Ta–N"1, Ta–N"2, Ta–N"3, Ta–N"4) with different Ta and N co-doped positions, and show that Ta or N addition could modulate the electronic, optical and photocatalytic responses of TiO_2 for photocatalysts applications. Results show that, Ta–O bonding not only reduces the electronic transition energies but also increased the carrier mobility of impurity level in Ta-doped and Ta–N"2, Ta–N"3, and Ta–N"4 co-doped TiO_2 specimens, while the impurity level disappears and the band gap reduces by 0.46 eV in Ta–N"1 co-doped specimen due to the forming Ta–N bonding. Most strikingly, Ta–N co-doped TiO_2 have twofold photocatalytic ability better than pure TiO_2 under visible light excitation. - Highlights: • The impurity level states are mostly decided by Ta–O bonding and Ta–N bonding. • The band gap reduced by 0.46 eV in Ta–N"1 co-doped specimen. • Ta–N co-doped TiO_2 have twofold photocatalytic ability better than pure TiO_2 under visible-light excitation.