To alleviate the load of the satellite and to deal with sudden trouble, research of co-aperture designing methods with which the space imaging optical system can be used as laser transmitting antenna was important. First, the co-aperture design method was proposed based on a given imaging optical system, and the optical coupling elements for meeting the imaging system to the laser transmitting request was analyzed when the laser transmitting system was based on EDFA. Then, the Gauss beam transmitting though the co-aperture system was analyzed, and the field distribution was given. Finally, the effect of distance changing between the coupling elements and imaging system on the transmitted laser were simulated. Result show that the co-aperture design can satisfy the imaging and laser transmitting request, and slightly adjusting the distance between the coupling elements and imaging system can change the transmitted waist and far-field peak energy without bad effect on the total transmitting energy, which make it possible for the system to satisfy different communicating mission.