A continuous-wave cavity ring-down spectroscopy (CW-CRDS) technique employing a broad-band diode laser is developed for the high reflectivity measurement. The theory of square-wave modulated CW-CRDS is presented. Both amplitude and phase shift of the first harmonic of the CRDS signal, measured at an appropriate frequency range, are detected by a lock-in method and fitted to obtain the ring-down time and the reflectivity. The measurements are repeated with different cavity spacings and the reflectivities are determined by the frequency-domain fitting methods. The reflectivity of the cavity mirror is determined to be 99.70%±0.01%. The agreement among the reflectivities determined with the frequency-domain fitting techniques indicates the high reliability of the CW-CRDS technique. Compared with the pulsed CRDS approach, the CW-CRDS technique is simple, low-cost and highly precise due to the use of a CW diode laser with high beam quality.