A new type of bidirectional coherent optical communication system is proposed combining unidirectional optical signal amplification. We analyze the application of discrete Raman amplifier (DRA) on the basis of dispersion compensating fiber in this system, and optimize the amplifier configuration considering the influence factors such as Rayleigh backscattering noise and amplified spontaneous emission noise. In addition, the gain and pump efficiency improvement in a novel design of double-pass DRA. The feasibility of the single-fiber bidirectional coherent optical communication system is ensured by the theoretical analyses and MATLAB coding. The system greatly decreases the phase shift problem caused by the frequency offset between the transmitter and local oscillator as well as the laser linewidth. It has practical value for secure communication systems.