A novel TiO2-wrapped nanofiber composite catalyst, which possessed a unique porous structure and mixed crystalline phase, was prepared by the combination of superficial sol–gel and post-calcination processes. By means of the superficial sol–gel process, TiO2 layers were deposited on the surface of each nanofiber-like cellulose fiber, and then the TiO2-wrapped nanofiber composite catalysts were calcined at different temperatures under a nitrogen atmosphere. With temperature increasing, the original cotton nanofiber composites were converted into porous carbon nanofiber catalysts wrapped by a TiO2 mixed crystalline phase, which was accompanied by a crystal transformation. The photocatalytic activity of the new catalysts was evaluated by the degradation of methylene blue (MB) under ultraviolet (UV) irradiation. The results demonstrated that the new catalysts had good photocatalytic ability, and the TNC-700 catalyst showed a superior photocatalytic ability compared with the other catalysts; the new catalysts had a unique porous structure, high specific surface area, and mixed crystalline phase. Additionally, the synergistic photocatalytic effect of the TiO2 and activated carbon nanofiber resulted in the efficient degradation of organic pollutants in water or air.