Abstract: According to the alignment demand of the large aperture aspheric mirror(hereinafter to be referred as the main mirror), the common detection methods for the large aperture aspheric surface were comparatively analyzed, and the best method for this sort of surface shape detection was put forward. In the alignment procedure of the main mirror, the two key links of the main mirror's fixation and deformation compensation were expounded, the difficulties of the fixation and the reasons for the deformation were summarized, and then a fire-new set of project was established by using measurement method with circumrotation counteracting gravity, real-time center position controling with particular tools, assistant material jointing method for the main mirror fixation and finally adjusting fixation with assistant upholders. The result showed that the method obtained good experimental effect, and the surface shape error of the main mirror was less than or equal to 0.03（＝632.8nm）.