A new measurement trace gas concentration system of Continuous Wave (CW) Cavity Ring-down Spectroscopy (CRDS) based on the effects of Sagnac and the measurement principle of cavity ring-down is proposed. Thefiber loop based on the Sagnac effects served as a reflector and formed a resonator with a high-reflectivity reflector. It can achieve the reflectivity of the resonator tunable, therefore, the requirement of the intensity of incident light is decreased and condition of the signal processing is provided. On this basis, the theoretical analysis and derivation of fiber loop’s Sagnac effect and the measurement principle of cavity ring-down are proposed. Meanwhile, we can obtain: the relationship between the reflectivity and fiber loop rotational speed, the formula of measurement of the trace gas concentration based on CWCRDS, and the influence of the fiber loop rotational speed on the measurement system of trace gas concentration. Finally, the various performance indexes of the system are proved via the simulation experiments.(20070216004)