This paper presents a passive buffer system of connection mechanism for space-assembled segmented mirrors. The purpose of assembling telescope in space is to overcome the limits of launch volume and mass to support ever increasing aperture sizes. Using this buffer system, the docking impact between two mirror segments can be alleviated effectively. In this paper, first, a design methodology called "double-elastic contact model" is developed. Then, the design details and operation principle of the connection mechanism, are presented to validate the passive buffer system. Finally, by comparing the traditional docking system, the simulated results of dynamic performance show that the proposed buffer system can absorb the impact energy effectively and is considerably suitable for assembling the space optical system.