Abstract: Online PD measurement technology is an useful method to find out insulation defect in switchgears without influencing reliabiltiy of power supply. The mechnism of this technique is described and its application to Guangzhou distribution system is studied. Analysis of four classical cases shows that it is necessary to carry out the research on online PD measurement technology in switchgears and get further study on developing tendency and earlier fault alarming method furtherly. In addition, the statistical characteristics between index value and all factors, such as enviroment temperature and relative humidity, is built. In the end, the variation characteristics in each phase of PD is studied, and fault early warning method that is suitable for practice is proposed.