AIMS is an infrareds optical system for the accurate measurement of solar magnetic field, which is a national major scientific research project currently developed. The guiding optical system of AIMS is an off-axis Gregorian system, due to the designed minimum angle between the optical axis of the optical system and horizontal plane is 14.036°, a sub-aperture stitching test approach is developed to test the wavefront of the system. That makes the process of precision alignment of the system very difficult and laborious. Therefore, we developed a two-step alignment approach that based on merit function regression method, the developed method can make alignment of AIMS guiding optical system efficiency and accuracy. In this paper, we explain the detailed two-step alignment method and apply it to the real alignment of AIMS guiding system. Aided with sub-aperture stitching measurements, the AIMS guiding system is aligned and the results show that in 0.076λrms wavefront error in effective aperture was achieved. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.