Insulating pull rod is a key component of circuit breaker and disconnector of gas insulated metal enclosed switchgear (GIS), which is used to transfer movement from the grounding part to the high voltage part. It needs to withstand the recovery overvoltage and frequent mechanical operation, which puts forward high requirements on the comprehensive performance of the glass fiber reinforced epoxy resin (GFRP) composite. However, many internal breakdown and disassembly failures of insulation rods occurred in recent years, which seriously threaten the reliability of power equipment. In this paper, the influence of moisture content on the electrical tree growth characteristics of GFRP was studied. Combined with the dielectric properties and the hydrolysis characteristics of GFRP with different moisture contents, the mechanism of the electrical treeing process of GFRP was explained from the perspective of electric field distortion and insulation degradation.