The dual-loop carbon monoxide concentration measurement system is introduced based on the sensing principle of fiber loop ring-down spectroscopy technology.The important role of erbium fiber amplifier and the influence of its gain fluctuation on the system are analysed.According to the feedback gain control of the erbium fiber amplifier,stability and precision of the system are further enhanced.The ring-down curves corresponding to different CO concentrations are obtained through experiments.Then,the absorption characteristics of different concentrations and the ralationship between ring-down time and concentration are also analysed,all of which provide an important reference for the high accuracy gas concentration on-line measurement.