A 1.23 m aperture optical telescope and its Coude optical system were introduced. Compared several forms of Coude optical systems with internal and overseas groundbased telescope, a kind of optical system was designed that used offaxis elliptical mirror as reimaging element. Then the corresponding measurement and alignment methods of optical system were obtained. After actual measurement and alignment, the wavefront errors among three processes (designed errors, test error and alignment errors) were compared and analyzed, which proved that the method of measurement and alignment were reliable and advanced. The focal length of Coude optical system is 125 meters, field of view is 1.5′, the band of wavelength is between 0.4~5 μm, and the designed RMS wavefront error is less than λ/50(λ=0.632 8 μm). The RMS wavefront error that measured in the room is less than λ/20. The telescope was used to observe fixed star, the wavefront was tested with ShackHartmann wavefront sensor, and the whole Coude optical system RMS wavefront error of less than λ/4 was obtained.