Following with the “high-resolution upsurge” appeared in many counties in recent few years, it is an inevitable trend to increase the size of the Optical Telescope. However, because of the volume constrains of space-borne astronomical instruments, segmented reflector is thought as the main measure of future astro-physical missions by many scientists. In this paper, a coaxial three-mirror anastigmatic system (TMA) with a segmented primary mirror is modeled in optical software. The optical system, which has 2.4m aperture, 48m focal length and the field-view angle of 0.3°×0.06°, works in the 450nm~900nm wave band. The ‘1+6’ aperture-stiching model is applied. Firstly, the initial structure of the system is inputted to the CODEV, and a certain constraint functions are set, and then the system automatically optimizes. Finally, designing results show that the Modulation Transfer Function (MTF) is really very near to the limit of diffraction. We get a good image quality of the optical system design results.