High-temperature lead-free dielectric ceramic capacitors are urgently needed in modern advanced power electronics systems. However, it is still a great challenge to realize both ultrahigh energy density (W_rec) and efficiency (η) under the harsh environment. In this work, the innovative 0.9(Sr_0.7Bi_0.2)TiO₃-0.1Bi(Mg_0.5Zr_0.5)O₃ (SBT-BMZ)-modified BaTiO₃ (BT) relaxor ferroelectric ceramics are ingeniously designed and synthesized via a solid-state reaction route. Optimal energy storage performances are attained in 0.65BT-0.35(SBT-BMZ) ceramic with an excellent W_rec of 4.03 J/cm³, couple with an ultrahigh η of 96.2% at 370 kV/cm, which are superior to other lead-free BT-based ceramics. The energy storage performances of the 0.65BT-0.35(SBT-BMZ) ceramic also display superior thermal stability (20 ∼ 180 °C) with a subtle change of W_rec (± 8%) and η (± 1%), frequency stability (1 ∼ 500 Hz) and cycle stability (1 ∼ 10⁶ cycles) at 200 kV/cm. Moreover, the charge and discharge experiments indicate that the outstanding power density (P_D ∼ 62.6 MW/cm³) and ultrafast discharge time (t_0.9 ∼ 62 ns) are also achieved. All these features demonstrate that the 0.65BT-0.35(SBT-BMZ) ceramic is expected to be widely used in next-generation of wide-temperature dielectric capacitors.