Balanced photodetector is the core component in the high-speedcoherent optical communication system. According to the analysis of coherent detection principle, the relationship between consistency coefficient and signal-to-noise ratio of the balanced detection is obtained, the structure of the balanced detector is presented, and the test scheme about coherent detection is designed. Under the communication rate of 5Gbps, the common mode rejection ratio of the balanced detector is more than 27 dB, and the minimum optical signal power using balanced detection to realize coherent demodulation than single tube detection’s is improved by 8 dB, which verifies the feasibility of the balanced detector used for high speed coherent optical communication.