A tunable broadband terahertz absorber based on graphene metamaterial is proposed and numerically demonstrated. The absorber consists of three layers: the upper is the graphene metamaterial layer, the middle is the SiO2 layer, and the bottom is the metallic layer. Simulation results demonstrate that the proposed absorber achieves over 90% absorption in 1.11- 2.61?THz with a relative bandwidth of 80.6% when Fermi level ?c=0.7?eV. The peak absorption rate of the proposed absorber can be tuned from 20.32% to 98.56% by changing the Fermi energy of graphene from 0?eV to 0.7?eV. Additionally, the proposed absorber is insensitive to polarization and has high absorbance to wide incidence angles. Such design may have some potential applications in thermal imaging, thermal detecting, and stealth technique.