We propose a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate. The two switchable functions can be simply achieved by inserting a VO₂ film in the metamaterial design. Finite-difference time-domain (FDTD) simulation results show that the proposed metamaterial can convert x-polarized incident wave to y-polarized reflected wave when VO₂ is at metal phase, and convert x-polarized wave to circularly polarized wave when VO₂ is at insulator phase. The metamaterial performs well in the two functions, i.e., the same broad working frequency band and near perfect polarization conversion. The switching effect originates from the switchable Fabry–Pérot cavity length induced by the phase change of VO₂. We believe that our findings provide a reference in designing switchable metamaterials.