Abstract: The mathematic model of the relationship between the misalignment and aberration in the process of computer -aided alignment is presented. The misalignment state of three-mirror-anastigmat system was simulated with Zemax, and the alignment data was obtained. The corresponding relationship between the misalignment error and elementary aberration is analyzed, and the misalignment characteristic of three-mirror-anastigmat system is summarized. The alignment scheme is determined by the above analysis. In order to verify the alignment scheme, the three-mirroranastigmat system was aligned according to the value which was calculated in accordance with the alignment scheme. The result demonstrates that the alignment scheme is feasible and guidable to the alignment of three-mirror-anastigmat systems.