A novel advanced approach to continuous-wave cavity ring-down spectroscopy (CW-CRDS) based on a rapidly swept cavity is described, in which an optical cavity with super-high reflectivity cavity mirrors is used as a gaseous species pool of absorption spectra. The cavity is swept by PZT at 4 Hz repetition rate to shift the frequency of optical radiation stored in the cavity. A continuous-wave tunable diode laser frequency is synchronously scanned over a small range (-1 cm-1) at a low repetition rate. The transmitted cavity mode peaks are recorded. Absorption spectrum data is extracted from the envelope of the transmitted cavity mode peaks. High sensitivity absorption spectra of low CO2 content of 0.5 mbar near 6537 cm-1 and the weak absorption band (-10-27 cm-1/(molecule- cm-2)) near 6577 cm-1 were recorded using the novel scheme. The detection sensitivity is much higher than that of traditional absorption spectroscopy technique. It provides a novel, high-sensitivity, effective method for absorption spectrum measurements and microanalysis of gaseous species.