Abstract: To realize the fast, efficient alignment of of-axis thre-mirror anastigmatic (TMA) system, a computer aided alignment (CAA) method was proposed. First, the aberration characteristics in the misaligned of-axis TMA system was theoretically analyzed to determine the evaluation parameters in CAA. Second, the key alignment parameters to be adjusted were given by combining the misalignment characteristics with the actual situation of the optical assembly. Finally, the magnitudes and orientation of the alignment parameters were calculated with the measurement data and CAA model. In order to verify the method, an of-axis TMA system was aligned according to this CAA method. The wav-front error (WFE) of the center and extreme edge FOVs were measured by wav-front sensor. After only once calculation and alignment, the measured root mean square (RMS) WFEs were reduced from 0.32 to 0.067(=632.8 nm). The experiment results demonstrate that this CAA method is feasible.