According to the influence of arc discharge on safe operation of electrical equipment caused by quality defects or poor contact, the mechanism and detection methods of arc were analyzed. According to the characteristics of UV intensity radiation, an arc detecting system based on channel photomultipliers was designed. The captured arc discharge intensity was monitored, as a judgment basis of arc discharge degree, to realize the protection and control of electrical equipments effectively. The results show that the system can detect weak UV light with high sensitivity and linearity, and achieve the online monitoring of weak UV light detection.