Based on the imaging theory of the coaxial three-mirror optical system, a folded off-axis three-mirror optical system is designed through the aperture diaphragm off-axis and the flat mirror to fold the optical path. In the full field of view, the design value of the modulation transfer function of the optical system at the Nyquist characteristic frequency of 62.5 lp/mm is better than 0.58, and the imaging quality is close to the diffraction limit. Based on optical interference detection technology and computer-aided assembly and adjustment theory, the sensitivity matrix of primary aberration and optical component misalignment are established, and the aberration characteristics of the misaligned optical system are analyzed, and on this basis, a specific and effective optical precision assembly adjustment scheme is proposed. After the whole machine is installed and adjusted, the root mean square value of the wave aberration of the optical system in the whole field of view is better than 1/14λ(λ is wavelength). Using the camera to simulate the target at infinity, the image can be clearly imaged at the 62.5 lp/mm limit resolution of the detector, and the performance is excellent.