Sr_0.775Bi_0.15TiO₃ ceramics with linear-like relaxor ferroelectric behavior are promising dielectric energy storage materials. Improving breakdown strength (BDS) is key to optimizing its energy storage performance and expanding its application. Herein, a multi-scale synergistic optimization strategy is employed by introducing Cd²⁺ in Sr_0.775Bi_0.15TiO₃ ceramics to improve the BDS and energy storage performance, and the underlying mechanism of performance optimization is systematically investigated. At the nanoscale, first-principles calculations combined with electrical testing and structural characterization reveal that Cd²⁺ doping increases the ionic disorder, A–O bond strength, and the formation energy and migration barrier of oxygen vacancies. This inhibits oxygen vacancy transport and enhances electrical insulation. At the microscale, numerical simulations verify that the composition with appropriate doping exhibits a small and uniform local electric field. This decreases the breakdown probability. Meanwhile, Cd²⁺ doping enhances relaxor ferroelectricity. Consequently, the BDS is improved while maintaining low remnant polarization, and the optimized Cd_0.05Sr_0.725Bi_0.15TiO₃ ceramic exhibits excellent comprehensive energy storage performance with a high recoverable energy density of 5.16 J/cm³ and an efficiency of 92.65 % under 490 kV/cm. The performance possesses outstanding stability over a broad temperature range (21–150 °C), a wide frequency range (10–1000 Hz), and up to 10⁵ charge–discharge cycles. This sample also shows a high-power density of 115.02 MW/cm³ and an ultrafast discharge time of 0.046 μs. Therefore, Cd_0.05Sr_0.775Bi_0.15TiO₃ ceramic is promising for advanced pulsed-power capacitor applications, and this work provides additional mechanisms and strategic guidance for improving BDS and energy storage performance of linear-like relaxor ferroelectrics.