Using a distributed-feedback (DFB) diode laser as light source, a high sensitivity system of continuous wave (CW) cavity ring down spectroscopy was built. With the diode laser’s feature of current modulation and tuning, the ring-down transient and spectrum of cavity loss can be observed easily, and the repeated measurement error of system is about 0.65×10-6. The etaloning effects found in experiment were theoretically analyzed and experimentally researched in detail, then the external weak reflection of cavity was confirmed as the source of the effects. And the main reflection face was found through analyzing. Ways of software and hardware to eliminate the effects were put forward and testified. The results show that the fitting error of software is about 0.4×10-6, and after hardware improving, the standard error of cavity loss spectrum’s fluctuation decreases from 9.2068×10-6 to 0.4561×10-6.