Dielectric capacitors are increasingly utilized in applications such as electric vehicles, 5G communication infrastructure, and AC-DC conversion systems in smart grids, owing to their rapid charge-discharge response, excellent power density, and minimal energy loss. Despite these advantages, their energy storage capacity remains relatively limited compared to electrochemical energy storage devices like supercapacitors and lithium-ion batteries. Glass-ceramics, which consist of an amorphous glass matrix and in-situ crystallized ceramic phases, offer a promising solution for enhancing dielectric performance. Through high-temperature melting combined with a controlled crystallization process, the uniform dispersion of ceramic phases within the glass matrix can be achieved. These materials typically exhibit several advantageous properties, including low porosity, uniform grain size, and high density, all of which help suppress electric field concentration effects, thereby significantly improving their breakdown resistance. Furthermore, precise control over the crystallization process allows for the modulation of ceramic phase proportions, optimizing dielectric properties and reducing dielectric losses. As pulsed power devices continue to evolve towards greater miniaturization, integration, and lightweight designs, there is a growing need for dielectric glass-ceramics that simultaneously offer high power density and high energy density. In this study, a series of Tb³⁺-doped BaO-SrO-Nb₂O₅-based glass-ceramics with varying Tb³⁺ molar fractions were synthesized using a high-temperature melting method, coupled with a controlled crystallization process. The influence of Tb³⁺ doping on the crystallization behavior and grain size of the glass-ceramics was thoroughly examined. This study also systematically investigates how the concentration of Tb³⁺ affects the composition, microstructure, dielectric properties, breakdown strength, and energy storage performance of the BaO-Na₂O-Nb₂O₅ glass-ceramic system. X-ray diffraction and microstructural analyses indicated that the incorporation of Tb³⁺ ions did not significantly alter the dominant crystal phase, which remained the tungsten bronze-type Ba₀.₅Sr₀.₅Nb₂O₆ phase. However, appropriate Tb³⁺ doping significantly enhanced the crystallinity of the glass matrix and increased the relative abundance of the Ba₀.₅Sr₀.₅Nb₂O₆ phase. Concurrently, the doping process effectively inhibited grain growth in the glass-ceramics, leading to improved microstructural uniformity. Dielectric and breakdown strength tests revealed that, as the concentration of Tb³⁺ increased, both the dielectric constant and breakdown strength of the BaO-SrO-Nb₂O₅ glass-ceramics initially rose and then decreased. At a Tb³⁺ doping concentration of 3 mol%, the dielectric constant reached 106.1, while the breakdown strength peaked at 1364 kV/cm. Consequently, the maximum energy storage density achieved was 9.87 J/cm³, approximately 2.29 times that of the undoped sample (4.30 J/cm³). The observed performance enhancement can be attributed to two synergistic effects. First, low concentrations of Tb³⁺ serve as nucleation promoters, optimizing the crystallization process and improving the uniformity of the microstructure. Second, Tb³⁺ doping reduces the activation energy for interface charge, suppresses space charge accumulation, and mitigates local electric field distortions, thus significantly improving the breakdown strength. These findings provide valuable experimental insights for the design and development of high-performance glass-ceramic materials for energy storage applications.