This paper theoretically studies the design of a device for achieving asymmetric transmission of circularly polarized light. The device is a two-dimensional (2D) photonic crystal (PhC) heterostructure with a fully photonic bandgap, which is with triangular lattice air holes embedded in the germanium and silicon. Here, a line defect is introduced in the 2D PhC to form an optical waveguide structure that can achieve high forward transmittance. At the same time, a microcavity structure is designed to diverge the light, which is combined with the total reflection principle to suppress backward incident light, and therefore the asymmetric transmission of circularly polarized light is achieved. As a result, the asymmetric transmission of circularly polarized light at the telecommunication wavelength (1550 nm) with high forward transmittance (up to 0.726) is realized. As the circularly polarized light is a linear superposition of two orthogonally linear polarization lights with a fixed phase difference (π/2), the structure designed in this study can realize asymmetric transmission of arbitrarily linearly polarized light at the same time. Therefore, it has a wide range of applications, including quantum communication, information processing, and integrated optics.