We propose an ultrathin terahertz waveplate of bi-layer chiral metamaterial for cross-polarization conversion at asymmetric transmission. The chiral metamaterial is constructed with hybrid coupling plasmonic resonators of a concentric ring and a double-split ring. The terahertz metamaterial can efficiently convert the ${y}$y-polarized wave into the ${x}$x-polarized wave with the cross-polarized transmittance over 97% and the polarization conversion ratio of 99% in simulation. The asymmetric transmission parameter, defined by the difference between two opposite propagating transmittances, can be as high as 0.9. The operation frequency and efficiency are geometrically adjustable with the ring size by exploiting the hybrid coupling effect of electric and magnetic resonances. The presented metamaterial enables the functionality of the nonreciprocal terahertz waveplate with high isolation.