Abstract: Online monitoring for SO₂ and its trend has important significance for fault pre-warning and analysis of SF₆ electrical equipment. Based on ultraviolet fluorescence method, an online monitoring device is designed including SO₂ UV fluorescence optical path system, detection circuit, software and online lossless gas sampling system. The online lossless gas sampling system has many merits including automatic sampling, gas recovery, gas wastage, leakage and pollution prevention and multiple decomposition gases detection. Experimental test results and field operation show that proposed device can satisfy needs of online SO₂ monitoring of power equipment.