Cavity ring-down spectroscopy (CRDS) is a high sensitive absorption spectroscopy. The key technique is to cut off laser light sharply and detect the signal when the laser intensity inside the cavity reaches a certain value. When the laser frequency is resonant with the cavity, the CRDS can be realized utilizing the quickly decreased reflected light signal of cavity to trigger off the acousto-optic modulator controlled by TTL signal. As the laser light is interrupted, the ringdown signal can be detected with a fast respond detector. Labview is used to accomplish data acquisition and data fitting, and the fitting result of the ringdown time of the empty cavity is 2.54 μs. The method is simple and valuable, and can be used in mirror reflectivity measurement and spectra analysis.