Abstract: Affected by the factor of atmospheric turbulence and spatial optical coupling efficiency in the satellite-ground coherent optical, its extraordinarily difficult to achieve high data rate and high detection sensitivity of communication. In view of the difficulty of spatial optical coupling in the satellite-ground coherent optical communication link, the method to improve the detection sensitivity by using large area balance detector was proposed. Based on the analysis of the balance detection technologys principle and structure, the balance detector with a diameter of 100 m and a working speed of 5 Gbps was designed, and the simulation test platform of the spatial coherent optical communication was built. Under the communication rate of 5 Gbps, the direct detection sensitivity of the detector is -18.6 dBm, the coherent detection sensitivity is -43.89 dBm, and the common mode rejection ratio is above 38.5 dB. The test results verify the feasibility of the spatial coupling balance detector for coherent optical communication.