The development of modern radar technology has a great threat to the stealth of military targets such as aircrafts. Absorber can be used to enhance the stealth of military targets. In this paper, an ultra-broadband THz absorber based on graphene is proposed. The absorption rate over 90% is from 0.48THz to 1.82THz and the relative absorption bandwidth is 117%. Then the absorbing mechanism of the absorber is analyzed by the method of impedance matching. The results show the validity of the simulation. Moreover, the absorber has excellent properties of insensitive polarization and wide incident angle range.