Lead-free dielectric ceramics with high energy-storage density and efficiency are ideal energy materials for sustainable development of the enery resource. In this paper, (1–x)K_0.5Na_0.5NbO₃–xBi(Mg_0.5Ti_0.5)O₃ ((1–x)KNN-xBMT, x = 0.05, 0.10, 0.15, 0.20) lead-free relaxor ferroelectric ceramics are prepared by the traditional solid-state method. The effects of BMT on the phase structure, microstructure, dielectric properties and energy storage behavior of KNN based ceramics are studied. With the increase of BMT content, the crystal structures of (1–x)KNN-xBMT ceramics gradually change from orthorhombic to pseudo-cubic phase, and transform into cubic phase finally. The addition of BMT can suppress grain growth of the ceramics, resulting in the average grain size decreasing from 850 to 195 nm when x increases from 0.05 to 0.20. Dielectric properties exhibit that the Curie temperature decreases with BMT content increasing, and dielectric peak at Curie temperature is broadened due to the addition of BMT. In addition, ferroelectric properties demonstrate that the addition of BMT reduces the remnant polarization (P_r) and coercive field (E_c) of the ceramics. The results indicate that (1–x)KNN-xBMT ceramics transform from ferroelectric to relaxor ferroelectric phase. Based on the calculation of hysteresis loop, the best energy storage performance is obtained at x = 0.15, of which the recoverable energy storage density (W_rec) and the energy storage efficiency (η) are 2.25 J·cm^–3 and 84% at its dielectric breakdown strength of 275 kV·cm^–1. Meanwhile, the ceramic with x = 0.15 exhibits good stability in a frequency range of 1–50 Hz, with an energy density variation of less than 5%, and temperature stability in a range of 25–125 ℃ with change of less than 8%. Moreover, based on direct measurement, the energy storage density (W_dis) of the ceramic with x = 0.15 is 1.54 J·cm^–3, and the discharge time is only 88 ns. The research shows that (1–x)KNN-xBMT ceramics have a wide application prospect in the field of environmentally friendly capacitors with high energy storage density.