•Oxygen vacancy distribution and migration during the degradation were explored by cathodoluminescence (CL).•Core-rim structure originated from doping could effectively retard the degradation of the dielectric ceramics.•A novel model of degradation was proposed considering both core-rim structure and grain boundary. Mg, Ca and Dy co-doped BaTiO3 ceramics were prepared by conventional solid-state method. About 25% of grains possessed core-rim structure, which was caused by different Ca and Mg solutions. Cathodoluminescence, an efficient tool to obtain defect information with high spatial and spectral resolution, was applied to investigate the degradation mechanism of the ceramics. Oxygen vacancy concentration in the core is higher than that in the rim due to the higher Ca content in the rim. In the initial stage of degradation, oxygen vacancies mainly migrated within the core region. In the middle stage, oxygen vacancies migration in the grains with core-rim structure was confined to the grains, from the core to the rim. In contrast, oxygen vacancies in the grains without core-rim structure migrated across the grain boundaries to the cathodic region. Core-rim structure could delay oxygen vacancy migration to prolong the lifetime.