In this paper， a vanadium dioxide （VO2） based tunable broadband terahertz （THz） absorber is designed on the silicon plane， which is composed of a VO2 resonator and a metal layer， separated by a thin silicon dioxide （SiO2） dielectric layer. Numerical simulation results show that VO2 with high conductivity （30000 S/m） is at its metal phase， and when its absorptivity is greater than 90%， the 2.0 THz absorption bandwidth can be obtained. In addition， the perfect absorption is realized with absorptivity of 99.3% and 99.6% at 4.5THz and 5.8THz， respectively. In contrast， VO2 with low conductivity （100 S/m） is at its insulation phase， and the peak absorptivity in the corresponding broad absorption band is only 8%. Therefore， by altering the conductivity of VO2 in the absorber， one can switch between absorption and reflection and realize the dynamic tuning of absorptivity in a broad frequency band. In addition， the proposed absorber is polarization-insensitive under vertical incidence due to its structural symmetry. Moreover， the absorber maintains an excellent absorption performance over a wide incident angle range.