To improve the on-line detection method of SF6 insulation equipment, the theory of cavity ring down spectroscopy (CRDS) is expounded. The main decomposed products of SF6 were discussed and H2S was selected as the feature component for detection. Finally, the H2S detection device based on CRDS was designed, and the detection of SF6 purity and the detection of micro water content were added. This device can measure the purity of SF6, micro water and H2S content on-line in real time. This device has been run at the GIL experimental site of the UHV experimental base of State Grid for one year, and the data have been analyzed. It is proved that the test data of the device is accurate and reliable, and the running condition is good.