We present a transmissive all-dielectric terahertz (THz) metamaterial half-wave plate with a double-working-layer structure. One layer works as a half-wave plate to enable polarization conversion of the incident THz wave, and the other layer functions as an antireflection layer to improve the transmission. The device is made of pure silicon only and can realize a high-performance polarization conversion at the designed THz frequency. Numerical simulations have been performed to show how the polarization properties of the THz wave can be adjusted by the structural parameters of the metamaterial. With appropriate structural parameters, the transmission for cross-polarization can reach 90%, and the polarization conversion rate can reach almost 100% at the designed operation frequency of 1 THz in simulation. Several samples have been fabricated and characterized, and the experimental results show a cross-polarized transmission of about 80% and a polarization conversion rate of almost 100% and agree well with the simulations.