<p>Dielectric ceramics with high energy storage density under low electric fields is of great significance for advancing next-generation energy storage devices toward safer, more economical, and more compact designs. In this study, we propose a B-site ion co-doping strategy by introducing Mg²⁺ and Nb⁵⁺ into lead-free ferroelectric 0.6K_0.5Bi_0.5TiO₃-0.4Sr_0.7Bi_0.2TiO₃ (KBST) ceramic to regulate energy storage performance. A series of (0.6K_0.5Bi_0.5–0.4Sr_0.7Bi_0.2) Ti_(1-<i>x</i>)Mg_<i>x</i>/3Nb_2<i>x</i>/3O₃ (<i>x</i> = 0, 0.01, 0.02, 0.03) ceramics was designed and fabricated via solid-state reaction method. In addition, the influence of doping on phase structure, microstructure, dielectric properties, and energy storage behavior were systematically investigated. As a result, both the relaxor behavior and energy density were enhanced significantly compared with undoped ceramic. Notably, a recoverable energy density (<i>W</i>_rec) of 2.32 J/cm³ and an energy efficiency (<i>η</i>) of 79.3% are obtained simultaneously in <i>x</i> = 0.02 ceramic under a low electric field of 222 kV/cm, along with excellent temperature stability from 30 to 120&#xa0;°C and fatigue resistance after 10⁵ cycling. This work provides an effective material design strategy for developing high-performance lead-free energy storage ceramics suitable for operation under low electric fields.</p>