Reduced graphene oxide(RGO) assisted TiO_2 composite photocatalysts have attracted increased attention because of their high photocatalytic performance under both UV-and visible-light irradiation. However, the corresponding properties under the long wavelength irradiation is far from expected. In this study, a series of RGO samples with well-designed N impurity and optimized surface functional groups are prepared and employed during hybridization with TiO_2(named N-RGO/TiO_2). The coupling between the doped N atoms and surface functional groups is analyzed based on the resulting photocatalytic performances, and the specific mechanism is revealed by the EPR, IR and lifetime of the photoinduced electrons. After comparing the corresponding performances of the RGO/TiO_2 and RGO/N-TiO_2 specimens, the results show that the surface functional groups of the RGO serve as a precondition to achieve π-d coupling between the graphene basal plane and TiO_2 nanoparticles, while N atoms in the RGO enhance the resulting photoactivity under a long wavelength range(>510 nm). The findings provide a potential approach to promote the photocatalytic properties of graphene-based composite photocatalysts with the low-energy incident photons.