In order to investigate the effect of N-doping on the electronic structure of anatase TiO2,and discover the mechanism of band gap narrowing after N-doping, we have carried out first-principles calculations based on density-functional theory(DFT) for anatase TiO2 system. Studies of band structures and densities of states and densities of electron distributing show that Ti atoms and N atoms at the conduction band(CB) area, happens a kind of strong correlated interaction, which will lead electrons of Ti atom on the 3d orbit to move to 2p orbit of N atom, and the entire conduction band will move to the Fermi level. Band gap will narrow and the Red Shift electrophoresis will happen. Theoretical result has been compared with the experiment result.