The principle and key technology of balance detcetor for coherent optical communication were analyzed.A set of high-speed balance detection system was designed,and the coherent optical communications test platform was built indoors.Actual tests show that the balance detector can achieve the different rate of coherent detection,which can provide an essential support for coherent optical communication.Under 2.5 Gbps,the sensitivity can be obtained -26 dBm under the condition of direct detection using single pipe,where dynamic range is -26-3 dBm.When the power of local oscillator laser is 3dBm,coherent detection sensitivity can reach 46.5 dBm,where dynamic range is -46.5-3 dBm.