Constructing the hetrostructure is a feasible strategy to enhance the performances of photocatalysts. However, there are still some fundamental details and mechanisms for the specific design of photocatalysts with heterostructure, which need further confirming and explain. In this work, g-C3N4-based heterostructures are constructed with TiO2 in different ways, and their intrinsic factors to improve the photocatalytic activity are systematically studied by density functional theory (DFT). When g-C3N4 combines horizontally with TiO2 to form a heterostructure, the interaction between them is dominated by van der Waals interaction. Although the recombination of photo-generated electron-hole pair cannot be inhibited significantly, this van der Waals interaction can regulate the electronic structures of the two components, which is conducive to the participation of photo-generated electrons and holes in the photocatalytic reaction. When the g-C3N4 combines vertically with TiO2 to form a heterostructure, their interface states show obvious covalent features, which is very beneficial for the photo-generated electrons' and holes' transport along the opposite directions on both sides of the interface. Furthermore, the built-in electric field of g-C3N4/TiO2 heterostructure is directed from TiO2 layer to g-C3N4 layer under equilibrium, so the photo-generated electron-hole pairs can be spatially separated from each other. These calculated results show that no matter how g-C3N4 and TiO2 are combined together, the g-C3N4/TiO2 heterostructure can enhance the photocatalytic performance through corresponding ways.