Graphene, as an advanced material, has been widely studied in the design of metasurfaces. In the terahertz band, graphene will facilitate the reconfigurable ability of the metasurface, enabling wavefront modulation. In this case, the unit cell will be expected to operate with a large range of phase shift. This paper presents a terahertz unit cell design based on a graphene-metal structure for beam-scanning metasurface. The mechanism of graphene in phase shifting is discussed. A sub-wavelength unit cell of the metasurface is proposed according to the discussion. The simulated current distribution reveals the mechanism of graphene in resonance tuning. The graphene is DC biased to tune the resonance of the metal structure, achieving a phase shift of over 330° in 0.288 THz. The proposed graphene-based unit cell with a large phase shift can be used for beam-scanning applications, and it will be a promising choice for terahertz wireless communication and sensing.