Titanium dioxide is a typical photocatalyst in terms of green chemistry, and extending the excitation light to the visible light range has attracted great attention. In this work, we successfully synthesized TiO₂ nanocrystallites enriched in oxygen vacancies and traces of N-dopants by a nanocasting method from functionalized mesoporous carbon sieves CMK-3 and tetrakis(dimethylamino)titanium (TDMAT), in which the functionalized CMK-3 acted as both reactant and hard-template. The obtained TiO₂-600 was well characterized by means of XRD, BET, Raman scattering, XPS, HRTEM, and EPR. Results demonstrated that TiO₂ nanoparticles possessed uniform particle size (5.0 nm), anatase phase and large surface areas (228 m² g⁻¹) due to the space-confinement effect of mesopores in CMK-3. Rhodamine B (RhB) was used as a representative organic pollutant to evaluate the photocatalytic activity under visible light irradiation. Results showed that the as-prepared TiO₂-600 had enhanced photocatalytic activity compared to commercial P25 and the degradation rate was calculated as 4.8 times higher than that of P25 under visible light irradiation. The increased photocatalytic activity of the TiO₂ catalyst can be attributed to the synergistic effect between oxygen vacancies and N-dopants.