A TiO2-based catalyst modified with highly condensed g-C3N4 has been synthesized though a simple vacuum calcination method and applied for the photocatalytic degradation of azo dye acid orange 7 (AO7). The obtained catalyst was thoroughly characterized by an array of analytical techniques, among which X-ray diffraction, Fourier transform infrared spectroscopy, UV-vis diffuse reflectance spectra, and transmission electronic microscopy proved a successfully combination of the two composites. And elemental analysis revealed a C/N ratio close to the theoretical data of g-C3N4, indicating highly condensed g-C3N4 in the composited catalyst. The catalyst showed excellent photocatalytic activity toward degradation of AO7 under both visible and UV light, and in addition it had better photocatalytic performance than that of the catalyst prepared by calcination in air. The high photocatalytic activity could be attributed to two aspects: the suitable band gap positions for g-C3N4/TiO2 composite which could improve the separation efficiency of photo-generated electron-hole pairs and the high condensation degree of g-C3N4 resulting from the vacuum calcination which is advantageous for higher carrier mobility and lower HOMO levels.