Balanced photodetectors can suppress effectively common-mode noise produced by laser and atmospheric channel in the space coherence optical communication. The sensitivity of balanced detectors are obviously improved in the coherent detection. In order to explore the performance of balanced detector, noise mechanism of balanced photodetector was analyzed. The 2.5 Gbps and 5 Gbps theoretical limit of sensitivity was presented in the condition of incoherent and coherent detection. Balanced photodetectors were designed and test system was built for the mesaurement of sensitivity. In 2.5 Gbps communication rate, incoherent detection sensitivity reached -21.1 dBm, and coherent detection sensitivity reached -41.2 dBm. Test results roughly coincide with theoretical analysis. The reason that test results deviates from the theoretical value was analyzed. Sensitivity analysis and design of balanced photodetectors provide a theoretical basis for the realization of the space coherent optical communication system.