9BT1BNTss–x Nb2O5–y MgO ceramics, with a molar ratio of 1: x: y, were synthesized, and their microstructure, bonding, and dielectric properties were analyzed (9BT1BNTss refers to a solid solution containing 0.9 mol BaTiO3 and 0.1 mol Bi0.5Na0.5TiO3). The results showed that all samples sintered at 1200 °C for 2 h exhibited core-shell structures with tetragonal and cubic phases, as well as secondary phases of Ba4Ti13O30 and Ba6Ti14Nb2O39. In this study, all five formulation samples (x = 0.02–0.04; y = 0.005–0.015) had a dense structure, with average grain sizes ranging from approximately 441±19 to 594±79 nm. The dielectric properties of the 9BT1BNTss–0.04 Nb2O5–0.015 MgO sample are as follows: εr = 998 (25 °C) and 1118 (200 °C); tanδ = 1.41% (25 °C) and 33.33% (200 °C); and ΔC/C25°C = 10.98% (TC) and 12.09% (200 °C). These values met the Electronic Industries Association (EIA) X9R specifications.