We report a thin-film circular polarizer consisting of three metal-grid layers to be used with a photoconductive antenna (PCA) to generate terahertz (THz) circularly polarized (CP) radiation. The polarizer has a high transmission with a measured 3 dB axial-ratio bandwidth of 54.7% from 0.57 to 1 THz. We further developed a generalized scattering matrix approach to provide insight into the underlying physical mechanism of the polarizer. We revealed that the Fabry-Pérot-like multi-reflection among gratings enables the high-efficiency polarization conversion. The successful realization of the CP PCA can find widespread application, such as THz circular dichroism spectroscopy, THz Mueller imaging, and ultrahigh-speed THz wireless communications.