A novel tunable broadband metamaterial absorber/reflector is designed. Its structure consists of square single-layer graphene and a metal ground plane separated by a dielectric layer of SiO2. Due to the symmetry of the unit cell, the absorber is polarization insensitive when the light wave is incident vertically. By continuously adjusting the Fermi level of graphene, the surface conductivity of graphene can be tuned, and the working state of the proposed structure can be switched between the reflector and the absorber throughout the whole broad absorption band. The numerical simulation results show that the absorption bandwidth of the absorber reaches 4.13 THz at absorptivity larger than 90%, and the absorber maintains good broadband absorption performance within a wide incidence angle range of 65°. The proposed absorber can be widely used in high-performance terahertz devices, such as active camouflage, imaging, modulators and photoelectric switches.