The strong band-to-band visible light absorption obtained by changing the bandgap of photocatalysts is desirable but challenging for TiO2. In this paper, a mechanochemical pre-reaction and subsequent heattreatment process were used to create TiO2/g-C3N4 heterojunctions. Acid-treated H2Ti3O7 nanobelts and superior thin g-C3N4 nanosheets (CN) were ground evenly and further heat-treated to grow rhombic TiO2 in situ on the nanosheets. The heterojunctions exhibited a band gap with the absorption in visible light region. Heterojunction formation effective tunes the surface and electronic structures of the composite, resulting in significant decrease of bandgap. g-C3N4-based heterojunctions (5TCN) exhibited excellent H2 generation (4991 lmol/g/h) and NO removal. In contrast, a TiO2-based composite (95TCN) revealed efficient photo reduction of Cr(Ⅵ) which was 2 times of that of TiO2 sample and 22 times of that of CN. The photochemical reaction mechanism of TiO2 and g-C3N4-based composites was discussed with the ratio of TiO2 and g-C3N4. The excellent performance is ascribed to single crystal rhombic TiO2 nanoparticles grown in situ on g-C3N4 to form well-developed heterojunctions which accelerate the carrier transfer. These results inspire the electronic structure engineering of photocatalysts to improve visible light absorption and provide a magic strategy for excellent photochemical activities.