The alignment method of off-axis three-mirror anastigmat combines initial positioning and computer-aided alignment. During manufacturing, due to the limit of testing methods, the off-axis fabrication and off-axis angle cannot be measured precisely. The initial positioning based on the manufacturing off-axis parameter can bring large initial aberration, which can not be converged when using computer-aided alignment. The optical path is analyzed based on the auto-collimation principle, and the mathematic relationship between the position of off-axis mirror and the reflected image is derived. A system based on the relationship is given to measure the off-axis fabrication and off-axis angle, it includes the right angle plate of high precision, linear guides of high precision, the special center orientation tool, internal focusing telescope and self-standardized micro-flat optical tube. The measurement accuracy of off-axis fabrication and off-axis angle is ±0.05mm and ±10″ respectively.