The lattice parameters, charge populations, band structures, density of states and absorption spectra of P-doped anatase TiO2 are calculated using the first-principles based on the density functional theory. The results indicate that when the Ti atom is substituted for P atom, the volume of TiO2 decreases. When P atom substitutes for O atom or exists as interstitial atom, the volume of TiO2 increases. The substitutional P at Ti site and interstitial P are oxidized to different degrees, and the substitutional P at O site is reduced a little. The different three sites of P doping result in the increase of anatase TiO2 forbidden gap width and the introduction of local doping energy levels. The band gap increasing of P-doped anatase TiO2 is in the following sequence: interstitial P>substitutional P at Ti site>substitutional P at O site. The absorption spectra indicate that the substitutional P at Ti site cannot enhance the visible light absorption ability of the anatase TiO2, whereas the interstitial P strongly enhances the visible light absorption ability of the anatase TiO2. The interstitial P is probably an important reason for the experimental enhancement of the photocatalytic activity of P-doped anatase TiO2.