C-doped TiO2 thin films were prepared by magnetron sputtering and the influence of nitrogen on the films during the sputtering process was studied. The microstructure, composition, optical properties and morphology of TiO2 thin films prepared under different nitrogen flow rates were investigated by X-ray diffraction, Raman spectrometer, X-ray photoelectron spectroscopy, spectrophotometer and atomic force microscopy. The results showed that all the deposited films were mainly amorphous and some weak anatase phase was found in Raman spectra. Moreover, with the increase of nitrogen flow rate, the characteristic peak intensity of anatase decreased, which meant grain refinement. When the nitrogen flow rate was up to 4cm3/min, the N content in the C-doped TiO2 films was 3.54% with its optical band gap changing from 3.29 to 3.55eV. At the same time, the transmittance of C-doped TiO2 films increased obviously. It can be concluded that nitrogen flow rate can control the optical band gap and optical absorption properties of the C-doped TiO2 films effectively.