This paper proposes a plasmonic waveguide with a triangular-shaped graphene-coated nanowire integrated on the silicon substrate. The transmission properties of the fundamental graphene plasmon mode and their dependence on the geometric and physical parameters are fully investigated by the finite element method. The results show that the gap distance, vertex angle, corner radius, and chemical potential of graphene have significant influences on the modal transmission properties. By optimizing the design, the proposed structure can be used to achieve the deep-subwavelength modal field transmission with a propagation length of up to 10 μm and a normalized mode area of only about 10 ^-6. The proposed graphene plasmonic waveguide may offer a certain theoretical basis for the design of tunable nanophotonic devices.