Because of the continuously increasing detection ability in astronomy, polarization aberrations are playing increasingly important roles in the performance of astronomical telescopes. In this paper, the polarization aberrations of an unobscured off-axis astronomical telescope used to detect weak gravitational lensing effect are analyzed using polarization ray tracing. The diattenuation map and retardance map of each mirror for the telescope are obtained, and both its Jones pupil and amplitude response matrix are determined. It is found that the polarization aberrations of the telescope have an impact on both the imaging contrast and the spatial distribution of the point spread function (PSF). The optics ellipticity of the telescope is analyzed. The variations of optics ellipticity, which are dependent on the field of view (FOV), are induced by polarization aberrations. The maximum and mean variations of the optics ellipticity in all FOVs are 7.5×10 ^-3 and 2.7×10 ^-3, respectively. At the FOV [-0.0487°, 0.155°], the maximum ellipticity interpolation error increases from 1.2×10 ^-4 to 1.1×10 ^-3. Our analyses reveal that polarization aberrations should not be ignored and should be optimized in telescopes that require ultrahigh imaging performance, such as those used for detecting weak gravitational lensing effect.