Although dielectric ceramic capacitors possess attractive properties for high-power energy storage, their pronounced electrostriction effect and high brittleness are conducive to easy initiation and propagation of cracks that significantly deteriorate electrical reliability and lifetime of capacitors in practical applications. Herein, a new strategy for designing relaxor ferroelectric ceramics with K0.5Na0.5NbO3-core/SiO2-shell structured grains was proposed to simultaneously reduce the electric-field-induced strain and enhance the mechanical strength of the ceramics.The simulatation and experiment declared that the bending strength and compression strength of the core-shell structured ceramic were shown to increase by more than 50% over those of the uncoated sample. Meanwhile, the electric-field-induced strain was reduced by almost half after adding the SiO2 coating. The suppressed electrical deformation and enhanced mechanical strength could alleviate the probability of generation of cracks and prevent their propagation, thus remarkably improving breakdown strength and fatigue endurance of the ceramics. As a result, an ultra-high breakdown strength of 425 kV cm−1 and excellent recoverable energy storage density (Wrec ∼ 4.64 J cm-3) were achieved in the core-shell structured sample. More importantly, the unique structure could enhance the cycling stability of the ceramic (Wrec variation < ±2% after 105 cycles). Thus, mechanical performance optimization via grain structure engineering offers a new paradigm for improving electrical breakdown strength and fatigue endurance of dielectric ceramic capacitors.