Ceramic-based capacitors for energy storage devices require simultaneously high energy density and efficiency. Achieving high electric breakdown field based on linear dielectrics is crucial. Here, A-site Sm3+ doped perovskite Ca1-1.5xSmx□0.5xTiO3 ceramics with introduced A-site vacancies (VA) were prepared. All Ca1-1.5xSmx□0.5xTiO3 ceramics crystallize in an orthorhombic structure, with lattice constants a, b, and c linearly decreased. As a result of Sm3+ dopants and VA, the grain size decreased while the ceramics’ density was improved. The permittivity decreases from 176 (x = 0) to 135 (x = 0.1), but tanδ is effectively constrained (∼10−4). What’s more, the dielectric breakdown strength is significantly improved from 429 kV/cm (x = 0) to 547 kV/cm (x = 0.1) with dielectric linearity is maintained. The optimum energy storage density of 2 J/cm3 (x = 0.02) with ultrahigh energy efficiency of over 93.7 % is achieved, which are superior to many existing linear dielectrics and relaxor ferroelectrics. This work confirms the energy-storage enhancement through chemical modifications and microstructural engineering.