The influence of polarization aberration on detection accuracy and imaging quality in a spatial optical system containing freeform surfaces was analyzed. First， based on the Jones notation， a freeform surface off-axis optical system with fringe Zernike polynomials as the characterizing function was proposed. Second， the polarization aberration analysis model of reflective optical system with freeform surface was developed. The influence of fringe Zernike polynomial low-order coefficients on the polarization aberration distribution of the field of view off-axis optical system was analyzed. Then， by tracing the polarized light through full field of view and by comparing before and after the freeform surface was introduced， the freeform surface effected on three types of polarization aberrations， that is， phase aberration， retardance， and diattenuation， of the field-bias optical system were verified. Finally， a wide-field off-axis three-mirror optical system with freeform surface and the polarization aberration of the full field of view were analyzed. The analysis results show that the phase aberration distribution is consistent with the freeform surface sagittal height distribution， and the retardance and diattenuation caused by the freeform surface account for 52.5% of the retardance and diattenuation of the total system， respectively. Improving this polarization characteristic change is important for the design of very high-precision optical systems such as deep-space astronomical telescopes.