Terahertz technology has been applied extensively in the fields of imaging, sensing, and detection etc. Various terahertz functional devices based on artificial metamaterials, such as absorbers, filters, and polarization converters, have attracted much attention from researchers. By introducing various tunable elements in the design, such as graphene, photosensitive silicon and vanadium dioxide (VO2), a controllable metamaterial absorber has become a hotspot. The temperature-controlled phase transition of VO2 has great potential in the development of simple and controllable metamaterial absorbers. In this paper, using the phase transition properties of VO2, we propose a dynamically temperature-tunable dual-band ultra-thin terahertz metamaterial absorber, which is composed of gold-VO2 resonator on top layer, SiO2 dielectric spacer and continuous gold film on bottom layer. At room temperature, the phase-change material VO2 is in insulating state, and the proposed absorber achieves dual-band absorption with absorption rates of 94.3% and 99.6% at the resonance frequencies of 4.11 THz and 6.01 THz, respectively; as the temperature rises to 340 K, VO2 is in metallic state, and the two absorption peaks of the absorber are red-shifted achieving absorption rates of 95.8% and 93.4% at the new resonance frequencies of 2.55 THz and 5.29 THz respectively. The thickness of the absorber is only 1/19 and 1/45 of the minimum and maximum operating wavelengths, showing ultra-thin characteristics. Furthermore, the dynamically adjustable absorption mechanism is demonstrated by studying the surface current distributions at resonance frequencies for VO2 in different phase transition. In addition, as the proposed absorber has a centrosymmetric structure whether VO2 is in an insulating or a metallic state, it possesses the characteristics of polarization insensitivity for both TE and TM waves; meanwhile, it also shows good absorption for both TE and TM waves under wide-angle oblique incidence. The tunable dual-band ultra-thin absorber presented here is simple and easy to be fabricated, and has important application prospects in the fields of stealth and detection etc.