Spectral parameters of molecular absorption lines, including positions, intensities, pressure-induced shift and broadening coefficients, are basic data for various applications, such as greenhouse effect and the atmospheric environment, and gas detection in the interstellar space. Along with the fast development of laser techniques, the cavity ring-down spectroscopy (CRDS) becomes a widely applied technique used for molecular spectroscopy, owing to its high sensitivity and high precision. Combined with the application of precise line profile models, CRDS has been used to retrieve numerous reliable and precise spectral data. These results have been integrated in spectral databases, and even used in testing fundamental physical laws and constants. Here a review of the CRDS method and its application in precision spectroscopy of molecules is given, in particular, using frequency-locked lasers. The studies of the high overtones of carbon monoxide and hydrogen are presented as examples of such applications.