Ag0.91Sm0.03NbO3-modified Na0.5Bi0.5TiO3–BaTiO3 ceramics (0.94−x)Na0.5Bi0.5TiO3–0.06BaTiO3–x(Ag0.91Sm0.03)NbO3 (x = 0, 0.03, 0.06, and 0.09) were prepared by solid-state reaction method. The structural, dielectric, and energy storage properties of the ceramics were systematically studied. Our results indicate that all ceramics exhibit pure perovskite structure with dense microstructure. All the elements in the ceramic are homogeneously distributed. The sample with the doping level x = 0.09, possesses good temperature stability of the dielectric constant (Δe′/e′150 °C ≤ ± 15%) and low dielectric loss (< 0.02) over a wide temperature of 84–318 °C. A large energy storage density value of Wrec = 2.12 J/cm3, high efficiency of ƞ = 83% under a low electric field of 18 kV/mm, as well as excellent temperature/frequency stabilities were simultaneously achieved in the sample. This work provides a viable way to design high energy storage performance lead-free ceramics operating at low fields.