In this study, the development of solid/solid interface degradation of key insulating parts of air-insulated switchgear is systematically investigated under the combined effect of electric-thermal-moisture factors. Through multi-physical field simulation calculation, the high-field region of electric field distribution inside the switchgear cabinet is identified, which is mainly concentrated at the end face of the contact box, with the maximum tangential field strength of 2.86 kV/mm, which is prone to flashover, discharge along the surface, and other insulation failures. Based on the simulation results, the epoxy-silicon oxide insulation specimens in the high field strength region are selected to carry out the electric damp heat aging test, and the deterioration law of the insulating parts is systematically analyzed. The results show that during the aging process, the dielectric properties, insulation resistance, absorption ratio and polarization index of the insulating parts show significant changes, and the study reveals the deterioration mechanism of the epoxy resin insulating parts under the action of electric-heat-humidity multifactors, which provides important theoretical and experimental bases for the evaluation of the insulation performance of the switchgear cabinet and the diagnosis of the faults.