Abstract: Based on the vector wavefront aberration theory, the misalignment induced aberration's characteristic of Cassegrain optical system was analyzed in this paper. It is shown that a misaligned Cassegrain has three residual 3rd aberrations. The 3rd order spherical aberration is constant over the field and the 3rd order coma increases linearly with the field but the zero coma point no longer locates at the field center. Moreover, the astigmatism aberration field in system without symmetric contains two zero astigmatism point, neither of which is necessarily located on the field center of view. It has been demonstrated that a Cassegrain under assembly is only measured to have perfect performance on-axis but not aligned in any significant way, so the measurements of multiple field points for the Cassegrain are required in the process of alignment.