Abstract: Through the variation of sulfur hexafluoride(SF₆) gas, the abnormal high pressure in the system of substation equipment can be found. Electrical equipment in gas insulation switch(GIS) such as: high voltage switch, switch and various equipment would cause short circuit due to poor contact, increases in resistance value, heating, melting, arc, and continuous arc. Because SF₆ gas in these appliances may not be very pure (having impurity such as H₂O), in the appliance failure, SF₆ and H₂O would generate hydrogen fluoride gas under catalytic conditions. Through the monitor of the sudden change of the hydrogen fluoride gas, an early warning for the possible emerged failure of the system equipment of GIS can be realized.