This study investigates the influence of X-ray irradiation on the dielectric properties of epoxy/alumina (EP/AL2O3) composites. Isothermal surface-potential decay measurements were carried out under different tube voltages, irradiation times and ambient atmospheres. The results show that for a given tube voltage, the surface-charge decay rate decreases immediately after irradiation. However, in both air and SF6 the decay accelerates as the irradiation time is prolonged. Trap-level calculations reveal that at 225 kV/2.8 mA, 100 kV/6.3 mA and 50 kV/12.6 mA the trap depth falls from 1.080 eV (unirradiated) to 1.056, 1.054 and 1.057 eV, respectively. Concurrently, longer irradiation creates more shallow traps in both atmospheres, further speeding up surface-charge dissipation. These findings demonstrate that high-penetration, high dose X-ray inspections used for visual defect detection in power-insulation equipment do not noticeably degrade the insulating materials. On the contrary, they enhance surface charge decay and thus help to maintain electrical insulation safety.