La-F two-step co-doping TiO2 photocatalysts have been prepared by the means of in-situ doping method using tetrabutyl titanate, lanthanum nitrate, and sodium fluoride as the main materials. The prepared samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectra (UV-Vis DRS). The photocatalytic property under visible light was evaluated by the photo degradation of Methylene blue (MB). In addition, by means of electrochemical impedance spectroscopy (EIS), the carriers concentrations of co-doping samples were calculated through Mott-schottky equation to discuss the mechanism of two-step co-doping. The results demonstrated that all the samples exhibit homogeneous anatase TiO2, La-F co-doping led to larger lattice distortion and refined the grain size of TiO2. SEM results showed that the morphology of samples were spherical in shape but the aggregation degree was different with each other, and the two-step co-doping could aggravate the reunion of the sample particles compared with that of the co-doping samples because of the difference of their preparation process. The La-F co-doping also could cause the absorption band of samples shifted towards the longer wavelength side and the band gap decreased accordingly at the same time. Simultaneously, n-n heterojunction was formed between TiO2 and Lax-Fy-TiO2 in the two-step co-doping TiO2 samples. The build-in internal electric field caused by the different Fermi level of TiO2 and Lax-Fy-TiO2 played an important role in the directional migration of charges in the two-step co-doping samples, and reduced its recombination of photo-generated electron and hole effectively, and then enhanced the charge carrier concentration. As showed in the electrochemical impedance results, when the doping ratio of La and F in the two-step La-F co-doping sample were 1.5% and 10% respectively, the carrier concentration reached 1.5×1020/cm-3, and it’s degradation rate of methylene blue under visible light is 96% at 120min, which was 2.6 times of pure TiO2 on the same condition.