A series of polypyrrole (PPy)/titanium dioxide (TiO2) nanocomposites were prepared in different polymerization conditions by 'in situ' chemical oxidative polymerization. The nanocomposites were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy spectra (XPS), and UV-Vis diffuse reflectance spectra. The photocatalytic degradation of methyl orange (MO) was chosen as a model reaction to evaluate the photocatalytic activities of TiO2/PPy catalysts. The results show that a strong interaction exists at the interface between TiO2 and PPy, the deposition of PPy on TiO2 nanoparticles can alleviate their agglomeration, PPy/TiO2 nanocomposites show stronger absorbance than neat TiO2 under the whole range of visible light. The obtained PPy/TiO2 nanocomposites exhibit significantly higher photocatalytic activity than the neat TiO2 on the degradation of MO aqueous solution under visible and UV light illumination. The reasons for improving the photocatalytic activity were also discussed.