The electronic structures and absorption spectra have been studied with density functional theory plus U for isoelectronic impurities C/Si/Ge/Sn-doped anatase TiO2. Our theoretical results show silicon atoms can attract electrons from the surrounding oxygen atoms due to their greater electronegativity than that of titanium atoms which leads to increasing the energy of 2p orbitals of the surrounding oxygen atoms and then forming new acceptor impurity level composed of oxygen 2p orbitals near the top of TiO2 valence band. This new acceptor level can easily bind the photoexcited holes for a period of time, thus effectively preventing the recombination of photo-excited electron-hole pairs, which play a key role in achieving higher light photocatalytic activity than pure TiO2 for degradation of contaminants in water. We also systematically described the electronic structure and optical properties of the germanium doped system and then explained experimental observations of absorption spectra with red-shift at a suitable Ge dopant concentration. Especially, compared with pure TiO2, Sn-doped TiO2 has better photocatalytic activity because of the synergistic effect of Sn atoms and oxygen vacancies. Our theoretical calculations not only explained the key role of isoelectronic defect in preventing the recombination of photo-excited electron-hole pairs but also provided a new way for enhance of photocatalytic activity.