Graphene-based van der Waals heterojunctions can not only modulate the electronic properties of graphene but also retain the superior properties of the original monolayer. In this paper, the structure, electrical contact types, electronic and optical properties of graphene/C ₃N van der Waals heterojunctions are systematically investigated based on first-principles calculations. We find that there is a p-type Schottky contact of only 0.039 eV in the graphene/C ₃N van der Waals heterojunctions in an equilibrium state. The external electric field can adjust the interface contact type, specifically, from p-type to n-type Schottky contact, or from p-type Schottky contact to Ohmic contact. The vertical strain not only opens a nonnegligible band gap of 360 meV on the Dirac cone of graphene in graphene/C ₃N van der Waals heterojunctions, but also modulates the band gap of C ₃N in the heterojunctions. Moreover, both the doping type and concentration of the carriers can be effectively tuned by the applied electric field and the vertical strain. The increase in carrier concentration is more pronounced by the applied electric field. Comparing with the pristine monolayer graphene and monolayer C ₃N, the optical response range and the light absorption rate of graphene /C ₃N van der Waals heterojunctions are enhanced. Main absorption peak in the spectrum reaches to 10 ⁶cm ^–1. These results not only provide valuable theoretical guidance for designing Schottky-based graphene/C ₃N van der Waals heterojunctions devices, but also further explore the potential applications of heterojunctions in optoelectronic nanodevices and field-effect transistor devices.