Dielectric ceramic capacitors (DCCs) are highly desired for advanced electronic and electrical power systems owing to their ultrahigh power densities and fast charge-discharge speed. However, the low recoverable energy density (W_rec) of dielectric ceramic resulting from the low Weibull breakdown strength (E_b) has been a long-standing challenge. Here, we fabricated 0.8Na_0.5Bi_0.5TiO₃–0.2Sm_1/3Sr_1/2(Mg_1/3Nb_2/3)O₃ (0.8NBT–0.2SSMN) relaxor ferroelectric (RFE) ceramics display a greatly improved E_b of 480 kV/cm and largely enhanced W_rec of 7.3 J/cm³, far outperforming pure NBT ceramic. We demonstrate that the proposed multi-scale insulation regulation strategy via introducing SSMN with optimal content can effectively reduce grain sizes, increase the bandgap, and create a highly insulating second phase, leading to a high E_b. Additionally, the introduction of Sm³⁺ and Mg²⁺–Nb⁵⁺ dopants on the A/B-sites of the Na_0.5Bi_0.5TiO₃ lattice created disruptions in the long-range-ordered ferroelectric domains, leading to excellent RFE property. The high E_b and excellent RFE property led to the substantial improvement of W_rec. Else, an exceptional thermal stability of W_rec and efficiency (η) are obtained at 25–200 ℃ (W_rec ~ (1.77 ± 0.08) J/cm³, η ~ 82.9% ± 4.3%). This work offers a route for designing high energy storage performance relaxor ferroelectric ceramics for high-voltage dielectric ceramic capacitors.