Abstract: There have been multiple internal breakdown faults of the insulated pull rods during the field testing or operation of gas insulated switchgear(GIS). These faults can affect the engineering operation and equipment reliability, thus urgently necessitating the exploration of their underlying fault mechanisms.. In this paper, glass fiber reinforced epoxy resin composite(GFRP) for insulated pull rod of GIS circuit breaker is selected as the research object. The experimental conditions of electrical treeing degradation under alternating loads are set up by calculating the principal stress distribution characteristics in the insulated pull rod during the switching. The effects of different alternating loads on the growth characteristics of electrical tree in GFRP are studied. The results show that compared with congener load, the growth rate of electrical tree in GFRP increases, the average breakdown time decreases and the breakdown probability increases under alternating load. With the help of ABAQUS, it is found that there will be large stress concentration at the fiber-resin interface of GFRP under alternating loads and the local stress damage has reached the critical failure value of GFRP. In summary, the interactive evolution of mechanical damage and insulation degradation at the fiber-resin interface of insulation pull rod under alternating loads is the main cause of insulation breakdown, and it is also the improvement basis for experimental assessment and structural design.