The light response range, redox potential, and consequently, the photo-reactivity of a semiconductor photocatalyst for solar-to-hydrogen are intrinsically dominated by its electronic structure. Introducing chemical impurities, structural defects, and construction of heterostructures have been considered as the widely adopted strategies to modify the electronic structures of two-dimensional photocatalyst. Up to now, however, the synergetic effect of these strategies on the photocatalytic property of photocatalyst remains unknown. In our work, using first-principles calculations, we systemically studied the effect of non-metal (sulfur) doping, vacancy engineering and interface coupling, as well as their synergetic effect on the electronic structure of g-C3N4. Results indicate that the introduction of doped S atom or C (N)-vacancy in g-C3N4 could extend the light response range by narrowing the band gap or creating gap states, while the coupling effect at interface of g-C3N4/MoS2 is able to tune the band edge potentials of g-C3N4. Incorporation of these strategies, a g-C3N4 with appropriate band gap and suitable band edge potentials can be obtained. Moreover, we also found that these modification strategies can influence the spatial electron distribution corresponded to the band edges, which would be beneficial to inhibiting the recombination of photogenerated electron-hole pairs. Therefore, incorporating doping or defect engineering with interface coupling should be an experimentally attainable way to improve the photo-reactivity of 2D semiconductors for overall photocatalytic water splitting.