An all fiber coherent detection system is developed for target velocity measurement.The narrow line width laser output power is 100 mW.It′s based on Doppler etfect.The operation distance is from 0.1 m to 3 km.The system′s sensitivity achieves 10-13 W，It has advantage of low power consumption,stable and reliable performance,good spatial adaptability.The system can be used for spacecraft landing which needs to measure velocity.