Cavity ring-down spectroscopy (CRDS) is a high-sensitive laser spectroscopic technique capable of measuring concentrations of trace gases. We have demonstrated a new design of the CRDS spectrometer with a continuous wave (CW) laser. The ring-dwon signal is produced through blocking the incident CW laser by scanning the cavity length fast toward off-resonance iwth PZT (piezoelectric transducer). We have also measured an absorption spectrum of acetylene overtone transitions near 570 nm at the pressure of 2700 Pa, and the minimum detectable absorption coefficient has been found to be about $3{ imes}10^{-9}cm^{-1}$.