Terahertz polarization devices are an important part of terahertz optical systems. Traditional terahertz polarization devices rely on birefringent crystals, and their performances are limited by the material structures. In this work, we theoretically demonstrate that the metamaterial consisting of the medium and the periodic metal band embedded in the medium can control broadband polarization effectively. The transmission length of the subwavelength waveguide mode gives rise to a broadband transmission peak. The resonant cavity structure formed by the dielectric layer and the waveguide layer possesses a high transmission efficiency. By optimizing the metamaterial structure parameters, we design a high-efficient (>90%) quarter-wave plate over a frequency range of 0.90 THz–1.10 THz and a high-efficient (>90%) half-wave plate over a frequency range of 0.92 THz–1.02 THz. Besides, due to the anisotropy of the structure, the metamaterials with the same structural parameters can achieve the function of the polarized beam splitting with an efficiency of up to 99% over a frequency range of 0.10 THz–0.55 THz. Therefore, the designed metamaterial has a multifunctional polarization control effect, which has potential applications in the terahertz integrated polarization optical system.