The invention aims to provide a multi-telescope optical synthetic aperture imaging system and a design method thereof in order to solve the problems that the conventional single large-aperture optical system is difficult to process and manufacture, and that the synthetic aperture imaging system cannot reach a desired system index in practice. The designed three-sub-telescope refractive and reflective optical synthetic aperture imaging system consists of a light beam synthesis mirror, three afocal sub telescopes which have the same structure and are arranged in a central symmetry manner on the system entrance pupil face, and optical delay lines which respectively correspond to the afocal sub telescopes; the aperture, on the light beam synthesis mirror, of the common circumcircle of the exit pupils of the three afocal sub telescopes meets the requirement of the magnifying power of the afocal sub telescopes; the optical delay lines are arranged on parallel emergent light paths of corresponding refraction systems; each of the optical delay lines consists of two planar reflecting mirrors; and the light beam synthesis mirror is a rotation symmetric system. The multi-telescope optical synthetic aperture imaging system is compact in structure and rational in layout, and lays the foundation for implementation of engineering application of the synthetic aperture imaging technology.