Cavity ring-down spectroscopy (CRDS) was used widely in trace gas detection and mirror reflectivity measurement due to the characteristics of high sensitivity, high precision and simple device. A CRDS set was established based on the continuous wave. Pound-Drever-Hall (PDH) frequency laser locking technique was utilized to lock the frequency of the laser on the eigen frequency of the cavity, so as to achieve the goal of wavelength stability of laser. By observing the transmission signal with different frequency of switch signal bearing on the AOM, optimal cut-off frequency 60 kHz was gotten. Then the system was used to measure the reflectivity of the cavity mirrors. Compared with the measurement results without locking, the results indicats that the locking case has less drift and the error is compressed into one third.