A microstructure-level model of the core-shell-structured ferroelectric ceramics was developed through the voronoi tessellation random construction routine. Assuming the shell is linear dielectric and parameterizing the ferroelectric core with a classical and a modified hyperbolic tangent model, finite element method was applied to solve the resulting systems. Statistics of electric field spatial distribution indicate that increasing applied electric field leads to intensified field fluctuation while this effect is weakened through the incorporation with thicker shell. Energy density extracted from the as-calculated hysteresis loops possesses an optimal value with regard to the shell fraction. Calculation results show that enhancing the saturation polarization and lowering the remnant polarization of the core, as well as modulating the shell fraction within a favorable range, provide the most effective way to obtain high-energy-density ceramics. Reducing the core initial and the shell permittivity benefits the energy efficiency. The underlying mechanisms of the enhanced energy density and reduced energy loss in the core-shell ferroelectric ceramics were discovered to be the trade-offs between the lowered polarization and the weakened dielectric nonlinearity as a consequence of the dilution and depolarization of the shell. This model can guide the engineering of core-shell-structured ferroelectric ceramics for energy storage.