A digital signal processing, infradyne, polarization multiplex quartered phase-shift keying ultra high sensitivity coherent optical communication system is reported in this paper. The transmission bit-rate is 127 Gb/s and the payload bit-rate is 99.53 Gb/s. Sensitivity of -38.2 dBm is achieved in real-time back to back transmission. In this system, commercial devices are widely used and the design follows ITU-T recommendation. Therefore, the system has great prospects of application.