Free space optical communication based on modulating retro-reflector(MRR) is research hotspots of future wireless optical communication because of less power, compact structure and without Acquisition, Pointing and Tracking (APT) requirement at system based on MRR. Full-duplex free space laser communication system based on MRR and double-wavelength laser is proposed. The link budget between small satellite under low orbit and ground station and bit error rate(BER) of optical communication are calculated when Communication distance and rate are 300 km and 1 GHz. It can be found that the link budge is more than 5 dB, and BER is less than 10-15. The results show that the budge and BER meet the communication requirement. Then limitation of the emulation is also analyzed. In conclusion, the proposed system can be used between light-weight full-duplex laser communication of small satellite under low orbit and ground station. It may be one of the future development trends of space optical communication system.