Metasurfaces can realize flexible modulation of electromagnetic waves at the wavelength level. However, the reported functions of metasurface are usually fixed and cannot be changed, once its structural design is completed. The designed metasurface cannot meet the requirements for flexible regulation of terahertz waves. We find that the phase change material of vanadium dioxide can achieve a transition from insulating state to metallic state through thermal, electrical, or light excitation, and the phase transition of this material is reversible. Therefore, using vanadium dioxide to form a composite metasurface can achieve dynamic modulation of terahertz waves. In this study, we propose a terahertz metasurface with switchable broadband absorption and polarization conversion. The proposed metasurface is composed of a 9-layer structure stacked from bottom to top with a combination pattern of different dielectric layers. By adjusting the conductivity of vanadium dioxide, the designed metasurface can achieve flexible switching between terahertz wave absorption function and polarization conversion function. When the vanadium dioxide is in the metal state, the designed metasurface behaves as a broadband absorber with an absorption rate of more than 90% in a range of 6.32–18.06 THz and a relative bandwidth of 96.3%. When the vanadium dioxide is in the insulated state, the designed structure acts as a polarization converter in a frequency range of 2.41–3.42 THz, 4.78–7.48 THz, and 9.53–9.73 THz, respectively, with a polarization conversion rate of over 90%. We believe that this metasurface structure will have good applications in the fields of terahertz wave detection, terahertz switches, terahertz filtering, terahertz communication, and terahertz sensing.