In accordance with a three mirror off-axis freeform square aperture system, orthogonal Zernike polynomial in square area was created to analyze wavefront and receive aberration parameter. During computer-aided alignment, by collecting interferograms of several fields of optical system by means of autocollimating interferometry, aberrations could be obtained from those interferograms. Furthermore, the sensitivity matrix related to the incorrect parameters derived from optical design software and these parameters of orthogonal polynomials in square area were used to determine the incorrect parameter. The results show that the averaging RMS value of the system is 0.164 1 waves(632.8 nm) and the averaging MTF value of the system is 0.453 4 after iterations, which proves it is right, and it can be used in off-axis freeform reflected system with other types of aperture.