(1 − x)BaTiO3-xBi(Mg0.5Sn0.5)O3 [BTBMS, 0.02 ≤ x ≤ 0.12, x represents the amount of Bi(Mg0.5Sn0.5)O3 (mol.%)] ceramics that were prepared by a traditional solid state reaction technique. X-ray diffraction results demonstrated that BTBMS belongs to a homogenous solid solution. The composition with x = 0.1 has the best properties with a stable relative permittivity (εr ~ 2918), the thermal-stability of relative permittivity (Δε/ε25°C ≤ ± 15%) in a large range of temperature from − 56 to 248°C and low dielectric loss (tanδ ≤ 0.025) from − 54 to 125°C. The relaxation in the high temperature region is thermally activated, and the oxygen vacancies may be the ionic charge carriers. Moreover, energy storage efficiency reaches the maximum at x = 0.1.