For (Na0.5Bi0.5)0.7Sr0.3TiO3-based relaxor ferroelectrics, the early polarization saturation and low breakdown electric field (Eb) are considered critical bottlenecks. As a result, the further development of this kind of promising material is severely restricted. Along these lines, in this work, comprehensive energy storage performances were achieved by the Bi(Mg2/3Ta1/3)O3 modified (Bi0.5Na0.5)0.7Sr0.3TiO3 ceramics [abbreviated as (1-x)BNST-xBMT]. The restrained rapid polarization saturation and large difference in the polarization were observed by the domain relaxor behavior. The improved microstructures and decreased grain sizes further enhanced the Eb values of BNST-based ceramics. Consequently, the suppressing premature saturation polarization, large polarization difference and high Eb value synergistically contributed to the enhanced comprehensive energy storage performances [the recoverable energy density Wrec of >3.0 J/cm3 and efficiency η of >85.0%]. Impressive stabilities of temperature (20 °C–200 °C) and frequency (1 Hz–200 Hz) were also demonstrated for the corresponding ceramics. Our results pave the way for improving the energy storage properties and further promoting the development of BNST-based ceramics.