The reduced graphene oxide (RGO)-based composites have attracted intensive attention in experiment due to its superior performance as photocatalysts, but still lacking is the theoretical understanding on the interactions between constituents, and the connection between such interaction and the enhanced photoactivity. Herein, the interaction between the g-C3N4 and RGO sheets is systematically explored by using state-of-the-art hybrid density functional theory. We demonstrate that the O atom plays a crucial role in the RGO-based composites. Compared to the isolated g-C3N4 monolayer, the band gap of composites obviously decreases, and at higher O concentration, the levels in the vicinity of Fermi level are much more dispersive, indicating the smaller effective mass of the carrier. These changes are nonlinear on the O concentration. Interestingly, appropriate O concentration alters the direct-gap composite to indirect-gap one. Most importantly, at a higher O concentration, a type-II, staggered, band alignment can be obtained in the g-C3N4-RGO interface, and negatively charged O atoms in the RGO are active sites, leading to the high hydrogen-evolution activity. Furthermore, the calculated absorption spectra varying with the O concentration shed light on different experimental results. The findings pave the way for developing RGO-based composites for photocatalytic applications.