A alignment method for infrared optical system of Cassegrain was introduced. The alignment of the primary and secondary mirror was finished on the optics centration error measuring instrument and the infrared eyepiece was aligned by means of metal-turning centering .Then, on the optics centration error measuring instrument, the alignment of primary and secondary mirror and eyepiece was completed. As the system has a linear detector, an external oscillating mirror was adopted. The oscillating mirror is oscillating with the fixed frequency, so the detector forms a complete picture of the target. In this way, the adjustment of the detector can be completed easily. One kind of this optical system was finished by this method. The noise equivalent temperature difference and minimum resolvable temperature different were measured and demands of system for all technique targets were met.