Herein, a novel strategy for regulating the phase structure was used to significantly enhance the recoverable energy storage density (W_rec) and the thermal stability via designing the (1-x)[(Bi_0.5Na_0.5)_0.7Sr_0.3TiO₃]-xBiScO₃ ((1-x)BNST-xBS) relaxor ferroelectric ceramics. The incorporation of BS into BNST ceramics markedly increases the local micro-structure disorder, causing a high polarization and inhibiting polarization hysteresis for 0.95BNST-0.05BS ceramics, leading to a large W_rec of 5.41 J/cm³ with an ideal efficiency (η) of 78.5%. Meanwhile, transmission electron microscope (TEM) results further proved that the nano-domain structure and the tetragonal (P4bm) phase superlattice structure of 0.95BNST-0.05BS ceramics possess an excellent thermal stability (20–200 ℃). An outstanding W_rec value of 3.18 × (1.00 ± 0.03) J/cm³ and an η value of 74.500 ± 0.025 are achieved under a temperature range from 20 ℃ to 200 ℃. This work provides a promising method for phase-structure design that can make it possible to apply temperature-insensitive ceramic dielectrics with a high energy storage density in harsh environments.