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As consumer electronics and industrial control systems continue to evolve, the operating temperature range of capacitors is gradually increasing. Barium titanate-based ceramic capacitors are widely used in the field of high dielectrics, so temperature-stable barium titanate-based dielectric materials have been a hot research topic in the field of dielectric ceramics. The construction of a core–shell structure by unequal doping is an effective way to obtain temperature-stable dielectric materials. At the same time, this structure retains part of the highly dielectric tetragonal phase, and materials with overall high dielectric constants can be obtained. In this work, we prepared BaTiO₃-xNaNbO₃-0.002Gd₂O₃ (x = 1.0–6.0 mol%) as well as BaTiO₃-0.05NaNbO₃-yGd₂O₃ (y = 0–0.30 mol%) dielectric ceramics. On the basis of high-electronic-bandgap NaNbO₃-modified BaTiO₃ dielectric ceramics, a core–shell structure with a larger proportion of core phase was obtained by further doping the amphiphilic rare-earth oxide Gd₂O₃. By designing this core–shell structure, the temperature stability range of capacitors can be expanded. At a doping level of 5.0 mol% NaNbO₃ and 0.20 mol% Gd₂O₃, the room temperature dielectric constant ε_r = 4266 and dielectric loss tan δ = 0.95% conforms to the X8R standard (from −55 °C to 150 °C, TCC < ±15%); volume resistivity ρ_v = 10,200 GΩ·cm and breakdown strength E_b = 13.5 kV/mm is attained in BaTiO₃-based ceramics. The system has excellent dielectric and insulating properties; it provides a new solution for temperature-stable dielectric ceramics.