Abstract: The metal insert-epoxy interface of AC GIS/GIL post insulators is prone to introduce air gap defects during the production and operation process, which is considered as an important incentive of the cracknels and breakdown failure of post insulators. By designing a post insulator with air gap defects and establishing a cracking simulation platform, the defect evolution and cracking process of insulator insert-epoxy interface were studied. The research results show that the cracking process is mainly divided into three stages, namely, the stage of crack generation and propagation, the stage of cracking, and the stage of explosion. The partial discharge caused by the air gap defect will induce the generation and deterioration of the crack, resulting in the rapid fracture of the insulator in a very short time and the cracking of the insulator under the impact of multiple high-energy arcs, showing the characteristics of brittle cracking. At the same time, the key factors affecting the bursting of pillar insulators are analyzed through experiments. The bursting probability of insulators under AC voltage is 10%~20% higher than that under DC voltage. Furthermore, the phase field simulation model of crack propagation under the action of electro-mechanical coupling was further constructed, and the dominant mechanism of different burst stages of the pillar insulator was analyzed. In the crack generation and propagation stage, the electric potential energy plays the main role; In the cracking stage, the effect of electric potential energy gradually weakens, and the mechanical potential energy gradually increases and plays a leading role. When the mechanical potential energy reaches the critical point of material stress, the insulator will crack brittlely. Electric potential energy plays an important role in the initiation and propagation of cracknels. In the cracking stage, the effect of electric potential energy is gradually weakened, and the mechanical potential energy gradually increases and plays a leading role. When the mechanical potential energy reaches the critical point of material stress, the insulator explodes. The research results of this paper preliminarily reveal the discharge embrittlement mechanism caused by the air gap defect of the pillar insulator and provide a theoretical basis for the fault analysis and optimal design of the pillar insulator.