A computer-aided alignment procedure is described, based on an off-axis three-mirror system. The relationship between the tilt and decenter variables is analyzed and the primary aberrations introduced by the variables are calculated. By eliminating the correlated variables, the alignment solutions are determined and damping factor is not needed. The processes of the method are as follows: 1) the correlation among variables is obtained by calculating the condition number of the sensitivity matrix for optical systems. 2) the alignment variables and magnitudes are gotten by eliminating correlated variables. The results simulated by a computer show that it is theoretically feasible to determine the solution by only one iterative. It can not only meet the precision requirement, but also accelerate the convergence of alignment solutions. This method is realized by compensation among variables, so the variables are reduced and the time of alignment is saved.