Many decomposition detection methods of SF₆ decomposition, such as detector tube, gas chromatography, mass spectrometry, and electrochemical method have some shortcomings, for instance, signal overlapp, poor accuracy or reliability, and not being able to realize SF₆ decomposition on-line monitoring. Cavity ring-down spectroscopy (CRDS) technique is proposed to monitor SF₆ decomposition. By studying the SF₆ decomposition mechanism in electrical equipment, the characteristic decomposition products and the corresponding absorption spectrum are analyzed and chosen in terms of CRDS technique. Based on CRDS, an on-line monitoring system of SF₆ decomposition in SF₆ electrical equipment is designed by modular method, and key technologies to improve the accuracy and reliability are explained in details, including cavity matching and anti-jamming technique.