Abstract: In order to realize high-precision assembly and adjustment of alt-az off-axis beam expanding optical system, the 4D interferometer was used to add plane lens matching the standard plane mirror to achieve self-collimation detection, and a dynamic detection method was proposed aiming at the problem of the tube needing for rotating around the pitch axis in actual. According to the actual assembly results, a space rectangular coordinate system was established, and the quadrature error of the two axes was calculated by using the maximum deviation of spot in rotation process. The results show, using this self-collimation detection and dynamic detection method, the surface figure accuracy root-mean-square(RMS) of primary mirror is 0.028 8λ@632.8 nm, the wave aberration RMS of the optical system is 0.131λ@632.8 nm, and the error of the two-axis orthogonality is 2.06″.