A high-sensitivity system of CW-cavity ring down spectroscopy is built with a distributed-feedback diode laser as light source. In which, the incident laser is in resonances with the mode of passive optical cavity via tuning the diode laser′s wavelength, and the switching off of laser is carried via modulating the diode laser′s current. The CW-CRDS system with and without etaloning effects is measured, and experiment shows that theirnoise-equivalent detection sensitivity is 2.56×10-7 cm-1 and 1.27×10-8 cm-1 respectively. The N2 broadening halfwidth of N2O molecule at 6591.43 cm-1 is measured with the longitudinal mode spacing of cavity as step of wavelength scanning, the measured half-width at half-maximum of line is 0.0819 cm-1, and 0.0808 cm-1, then the different between measurement value and HITRAN2004 database is discussed.