The unrestricted control of circularly polarized (CP) terahertz (THz) waves is important in science and applications,but conventional THz devices suffer from issues of bulky size and low efficiency.Although Pancharatnam-Berry (PB)metasurfaces have shown strong capabilities to control CP waves,transmission-mode PB devices realized in the THz regime are less efficient,limiting their applications in practice.Here,based on Jones matrix analysis,we design a trilayer structure (thickness of ～λ/5) and experimentally demonstrate that the structure can serve as a highly efficient transmissive meta-atom (relative efficiency of ～90％) to build PB metadevices for manipulating CP THz waves.Two ultrathin THz metadevices are fabricated and experimentally characterized with a z-scan THz imaging system.The first device can realize a photonic spin Hall effect with an experimentally demonstrated relative efficiency of ～90％,whereas the second device can generate a high-quality background-free CP Bessel beam with measured longitudinal and transverse field pattes that exhibit the nondiffracting characteristics of a Bessel beam.All the experimental results are in excellent agreement with full-wave simulations.Our results pave the way to freely manipulate CP THz beams,laying a solid basis for future applications such as biomolecular control and THz signal transportation.