To improve photocatalytic activity of TiO2, 3-hexylthiophene monomers were in situ polymerized on porous TiO2 nanocrystalline thin film. Poly-3-hexylthiophene (P3HT) was homogenously modified on the thin film. The surface modification amounts of P3HT were controlled using different concentrations of 3-hexylthiophene monomer ether solutions and detected by the absorption spectra. The photocatalytic performance tested in methyl orange solution under ultraviolet light irradiation was significantly enhanced due to the modification of P3HT. Within 210 min, approximately 80% of methyl orange was degraded for the modified film with the optimized modification amount, it is twice higher than that of the film without modification. Photoluminescence spectra and open-circuit voltage decay processes of the samples were measured to demonstrate the photocatalytic activity enhancement mechanism due to the in-situ polymerization of P3HT. The homogenous modification of P3HT can promote separation of photogenerated electron–hole pairs on the TiO2 nanocrystalline thin film, which suppresses the recombination of photogenerated charge carriers, thus improving its photocatalytic activity.