Off-Axis optical systems do not have rotational symmetry and cannot be optically centering turned by CNC machine to eliminate component tilt errors, so they are more difficult to be aligned than the coaxial optical systems. In this paper, the computer-Aided alignment technology is carried out for a four-mirror off-Axis reflective optical system. Firstly, aberration characteristic analysis is performed to obtain the relationship between the degrees of freedom for mirrors and the wavefront coefficients. Using the results of the above analysis, a segmental alignment method is proposed. Finally, the article has completed the experimental verification for the optical system alignment. The wavefront quality meets the requirement which verifies the validity of the theoretical analysis. The research results can provide effective solutions for the alignment of various off-Axis optical systems. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.