Abnormal electrical breakdowns have been observed in the grid system of ion thrusters during both ground tests and space missions, leading to severe discharge-induced damage. Alumina ceramic, commonly used as an insulating material in grid systems, is subject to surface modification under ion beam irradiation, which in turn significantly alters its surface charging behavior. In this study, we systematically investigated the charging characteristics of alumina ceramics after Xe+ ion irradiation, including trap distribution, surface resistivity, and surface potential. Our results demonstrate that the dominant charging mechanism shifts from secondary electron emission to charge injection after irradiation. Prolonged ion irradiation results in increased surface negative charge accumulation, which increases the risk of insulation failure. Furthermore, a self-consistent electron transport model is employed to support the experimental measurement. This work aims to establish the intrinsic link between ion irradiation damage and insulation degradation, contributing to uncovering the underlying physical mechanisms of abnormal discharges in ion thruster grid systems.