Coaxial transmission optical systems can be assembled by optical centering processing and optical centering assembly to eliminate the eccentricity and tilt of each lens, thus ensuring the coaxiality of the lens. Non-coaxial transmission optical systems cannot be assembled by the traditional optical centering method, and this type of system poses a new challenge to the assembly method. Aiming at a non-coaxial transmissive optical system, this paper proposes a precise assembly method based on the spatial coordinate change and the principle of self-collimation imaging. Firstly, we calculate the spatial angle and eccentricity value of different axis mirror groups, and then use two self-collimating theodolites and optical reticle centering tools to construct the spatial angle reference and eccentricity reference for different axis mirror groups. Then, the spatial position of each lens group is determined by the adjustment method of auto-collimation. The article finally carried out the alignment experiment. The wavefront RMS of the optical system after alignment is 0.026λ@632.8nm which meets the design requirements and verifies the effectiveness of the proposed method. © 2023 SPIE.