Arc discharges accompany with ultraviolet (UV) radiation. By detecting the radiation intensity of arc discharge the damage degree of arc to electrical equipments can be predicted. The mechanism and characteristics of arc are analyzed. According to the arc mathematical model, detection methods of arc radiation and characteristics of discharge pulse caused by spicule and metal fracture are studied. A detecting system based on channel photomultipliers is designed to capture arc. The UV radiation intensity at different distances is measured as a judgment basis of arc discharge degree. The improvement of detection effect by using light guide and light filtering modules is illustrated. The results show that the system can detect weak UV light with high sensitivity and linearity, and realize on-line monitoring of weak UV light.