This paper presents a discharge fault of the insulation rod in a 126 kV gas-insulated switchgear (GIS) circuit disconnector compartment at a 110 kV substation. Through disassembly inspection, micro-moisture and gas composition analysis in the gas chamber, fault oscillography analysis, and electric field simulation, the fault characteristics and root causes are investigated. Corrective and preventive measures are proposed, providing significant reference value for operational maintenance personnel in equipment upkeep and fault handling.