Due to the problems of low absorptivity and the narrow bandwidth of existing broadband absorbers, a perfect broadband tunable absorber with high absorptivity based on double-layer graphene is proposed in this study with 99% high absorptivity as the reference standard, and its theoretical analysis is performed. The designed absorber consists of complete graphene, upper dielectric material, patterned graphene, lower dielectric material and the gold substrate. Through numerical calculation, the designed absorbing structure can achieve high absorption rate of more than 99% in the range of 2.54–3.70 THz, and has a continuous bandwidth of 1.16 THz. In addition, the absorption range is 52%–100% by changing the Fermi level of the graphene layer. The perfect absorber is symmetrical in rotation center and insensitive to incident angle and polarization. It can be widely used in terahertz switch, modulator design, power acquisition and other related fields.