This study proposes a tunable ultrawideband metamaterial absorber based on a graphene-vanadium dioxide composite metamaterial structure. Ultrawideband flexible and adjustable functions are realized using the electrically tunable properties of graphene and the phase-transition properties of vanadium dioxide. Under the premise that the absorption rate exceeds 90%, the absorption bandwidth of the absorber reaches 7.003 THz in the frequency range of 5.4491?12.4521 THz, and the relative bandwidth is 78.24%. The effects of the Fermi level of graphene, vanadium dioxide conductivity, incidence angle, and polarization angle on the absorption rate are also analyzed. Analytical results show that the proposed absorber has an ultrawideband, is insensitive to the incidence angle and polarization, and exhibits flexible tunability. The study also finds that the optimization of the performance of metamaterial absorbers is of great significance.