A broadband terahertz (THz) perfect absorber that can be dynamically adjustable is designed. The absorber consists of a cross-shaped vanadium dioxide layer, metal ground plane, and sandwiched silicon dioxide layer. Simulation results indicate that a bandwidth of 1.06 THz (ranging from 0.71 to 1.77 THz) provides greater than 90% absorption. The absorptivity varies with VO₂ conductivity and can be dynamically adjusted from 4% to 99.5%. To obtain the physical mechanism of the absorber operation, impedance matching theory and wave-interference theory are introduced, and the physical source of the two perfect absorption peaks is analysed by electric field distribution. The absorber has the characteristics of wide-angle absorption and polarisation insensitivity and can be used in THz sensors, detectors, and stealth equipment.