Polymer-based nanocomposites always exhibit excellent energy storage capacity and have a great potential to be used in the field of electrical equipment and electronic device. In this study, the Ba_xSr_1-xTiO₃ nanoparticles wrapped with SiO₂ (Ba_xSr_1-xTiO₃@SiO₂) are chosen as inorganic nanofillers and filled into the poly(methyl methacrylate) (PMMA) matrix. The intrinsic properties of inorganic fillers, including phase composition, dielectric properties, breakdown strength and polarization behavior, on the energy storage performances of polymer-based nanocomposites have been systematically investigated. Especially, the fillers with ferroelectric phase composition will be polarized when they are subjected to a local electric field, leading to the changed dielectric constant, this factor is considered to analyze the electric field distribution for the first time. The results show that the optimized phase composition of Ba_xSr_1-xTiO₃ enables the nanocomposites to possess synergistically improved breakdown strength and polarization, giving rise to the excellent energy storage performances, where an energy storage density of 19.6 J/cm³ and an efficiency of 74.4% for 1 vol% Ba_0.6Sr_0.4TiO₃@SiO₂/PMMA nanocomposites. This study provides a valuable way to select appropriate inorganic fillers and optimize energy storage performances of polymer-based nanocomposites.