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High-performance Sr1–x Er x TiO3 (x = 0–0.014) ceramics were sintered in different atmospheres using the conventional solid-state reaction method. The phase structure and micromorphology of ceramics were analyzed using X-ray diffraction and scanning electron microscopy. Meanwhile, the Sr1–x Er x TiO3 (when x = 0.012) ceramic sintered in hydrogen attains a colossal permittivity (132 543 @1 kHz, 157 650 @1 MHz) and ultralow tan δ (0.009 @1 kHz, 0.03 @1 MHz) and has good frequency stability (20 Hz to 2 MHz) and temperature stability (−180 to 425 °C). X-ray photoelectron spectroscopy, electron paramagnetic resonance, and impedance analysis show that the colossal permittivity and ultralow dielectric loss are attributed to the defect dipoles and defect clusters [TiTi ′–VO ••–TiTi ′], [ErSr •–TiTi ′], [2ErTi ′–VO ••], and [ErSr •–ErTi ′]. The insulation resistivity is determined by the grain boundary. The dielectric properties of samples sintered in hydrogen are excellent, and then, the oxidation method is used to backfill the oxygen vacancy (VO ••), thus improving the insulation resistivity (2.8 × 1014 Ω cm) of the grain boundary. In addition, the diffusion mechanism of ceramic VO •• from low, medium, and high temperatures was studied by monitoring VO •• behavior in real time. The results reveal that the diffusion coefficient of VO •• in the grain boundary is greater than that in the grain; as a result, as the external oxygen partial pressure rises, the VO •• escapes first from the grain boundary. When the external oxygen partial pressure decreases, oxygen atoms enter the grain boundary region first and backfill oxygen vacancies.

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