Dielectric ceramic capacitors have attracted increasing attention as advanced pulsed power devices and modern electronic systems owing to their fast charge/discharge speed and high power density. However, it is challenging to meet the urgent needs of lead-free ceramics with superior energy storage performance in practical applications. Herein, a strategy for the composition and structural modification is proposed to overcome the current challenge. The lead-free ceramics composed of BiFeO3-SrTiO3 are fabricated. A low hysteresis and high polarization can be achieved via composition optimization. The experimental results and finite element simulations indicate that the two-step sintering method significantly influences the decrease in the grain size and improvement in the breakdown strength (EBDS). A high EBDS of ≈750 kV cm−1 accompanied by a large maximum polarization (≈40 µC cm−2) and negligible remanent polarization (<2 µC cm−2) contribute to the ultrahigh energy density and efficiency values of the order of 8.4 J cm−3 and ≈90%, respectively. Both energy density and efficiency exhibit excellent stability over the frequency range of 1–100 Hz and temperatures up to 120 °C, along with the superior power density of 280 MW cm−3, making the studied BiFeO3-SrTiO3 ceramics potentially useful for high-power energy storage applications.