Both space optical system and laser communicating antenna need a large aperture optical system, which increases the load of a satellite. This paper proposes a new co-aperture optical system for space imaging and laser communication to decrease the mass of the satellite. On the basis of primary aberration theory, the design method of initial configuration of the co-aperture optical system was presented, in which the imaging and communication share primary and secondary mirrors and work at different fields. A co-aperture optical system was designed with a primary mirror with a diameter of 600 mm and a secondary mirror with a obstruction ratio of 0.225. The system shows good imaging capability and its Modulation Transfer Function(MTF) for all field of view is more than 0.47, very close to the diffraction limitation. The RMS wavefront of the laser sending section is far less than λ/20, and the sending laser beam is in good quality with a least divergence angle about 4 μrad.The size of the light spot on the imaging surface is less than that of detecting surface, which satisfies the detecting request. Finally, the tolerance analysis was preformed and the assembling method was given. By sharing the primary and secondary mirrors, this design reduces the mass and volume of the system and satisfies the needs of space imaging and laser communication.