In the mid - 1960 - s, the birth of GIS, makes the high voltage power equipment qualitative leap. In recent decades, the GIS with its compact structure, space and building cover an small area, the advantages of the high reliability and easy installation, obtained the rapid development. At the same time, the safe and reliable operation for a long time also become the focus of attention. Studies have shown that, GIS has internal SF6 gas leakage, switch fault, internal discharge hidden trouble. In all kinds of fault, large of the failure caused by the insulation defect. Therefore, on-line insulation monitoring of GIS is very necessary.