A high-sensitivity system of continuous wave cavity ringdown spectroscopy (CW-CRDS) is presented, which uses an ultralow-thermal expansion glass-ceramic as the cavity and a Distributed-Feedback (DFB) laser diode as the light source. In the system, the laser frequency overlaps with one of cavity modes via scanning the cavity length. And the input laser is switched off or on by modulating the injection current of laser diode. Furthermore, the laser wavelength is scanned by tuning its operating temperature or injection current. The noise-equivalent detection sensitivity of 7.6×10-9 cm-1 is achieved in 24.9 cm cavity length. With an ultra-high vacuum system, the intensity and doppler broadening line-width of 19 spectral lines of N2O molecule in the range of6586.5～6596.5 cm-1 are measured, and then the difference between measurement results and that of Hitran2004 database is discussed.