We propose a tunable terahertz broadband absorber based on circular and L-shaped couplings. Its main structure is composed of an electrically tunable graphene material at the top, a dielectric material SiO₂ in the middle, and gold at the bottom. The absorption spectra of this device are studied using the COMSOL software and the multiple reflection interference theory. The results show that the simulated and theoretical absorption spectra significantly overlap. On the premise of keeping the slit, changing the shape of the central graphene will not change the performance of the absorber. When the Fermi energy level of graphene is regulated to 0.95 eV by an applied voltage, the device is polarization independent and exhibits a strong absorption of more than 90% with a spectral width of more than 4.1 THz. This study will therefore have potential applications in optical switching, modulators, and energy harvesting, and it will provide further inspiration for the design of terahertz broadband absorbers.