•C dopant improves the transformation of anatase-to-rutile phase.•Carbonate species and coke-like structure cause a absorption in visible -light region.•C-TiO2, prepared with G/T ratio 1 and 400°C, shows optimal photocatalytic activity.•The C existed at interstitial positions of TiO2 lattice induced the red shift.•The prepared C-TiO2 enables effectively degrade ethylene under visible -light irradiation. A C-doped TiO2 photocatalyst has been found to be viable in degrading organic compounds under visible light. However, the characteristics of the C-doped TiO2 in relation to photocatalytic activity remain largely unexplored. In this study, we systematically investigated the effect of carbon content and calcination temperature on the photocatalytic properties of a C-doped TiO2 catalyst prepared by a sol–gel method. The phase, structure, chemical state, optical properties, and surface area/pore distribution of the C-doped TiO2 were characterized using X-ray diffractometer, transmission electron microscope, X-ray photoelectron spectroscopy, UV–visible diffusion reflectance spectroscopy, and specific surface area by the Brunauer–Emmett–Teller method. The results showed that the photocatalytic activity of C-doped TiO2 was greatly enhanced compared to pure TiO2 under visible irradiation. The C dopant retarded the transformation from anatase to rutile. Namely, C doping was shown to stabilize the anatase phase. The results also showed that C atoms were incorporated into the interstitial positions of the TiO2 lattice or formatted coke-like structure on the surface of TiO2. The formation of carbonate species could cause a long-tail absorption in the visible-light region. Ethylene was used to evaluate the photocatalytic activity of C-TiO2 under visible-light irradiation. The results suggested that crystalline structure and smaller particle size enhance the photocatalytic activity of C-doped TiO2. The highest photocatalytic performance was found for a C-doped TiO2 catalyst prepared at glucose to titanium n-butoxide (G/T) molar ratio of 1.0 and calcined at 400°C.