Visible-light photocatalytic hydrogen evolution is a key step towards utilizing solar energy. Here we constructed covalently bonded oxidized graphitic C3N4/TiO2 (O-g-C3N4/TiO2) heterostructure with high surface area via an in-situ solvothermal synthetic strategy. The edge-anchoring of two-dimensional (2D) TiO2 on 2D O-g-C3N4 was evidenced by the HAADF-STEM EDS mapping, EELS and XPS analysis. Density functional theory computations suggest strong affinity between TiO2 and O-g-C3N4 2D structures through N-O-Ti covalent bonding, which drastically enhances the synergetic effect of the light absorption of O-g-C3N4 and high surface area of TiO2. The as-formed heterojunction can remarkably boost the visible-light photocatalytic activity for H-2 evolution by 6.1 times compared to physical mixture of TiO2 nanosheets (NS) and O-g-C3N4.