High-performance dielectric capacitors are in high demand for advanced electronics and electric power systems. They possess high power density (on the order of Megawatt) and exhibit ultrafast charge/discharge capability (on a microsecond scale) and long-term storage lifetime1-5, and thus they are particularly demanded in pulse power systems such as high-power microwaves, hybrid electric vehicles, and high-frequency inverters. However, their relatively low operating temperature limits their widespread applications6-9. Here, guided by phase-field simulations, we synthesized capacitors with an energy storage density of 55.4 joules per cubic centimeter, energy efficiency of over 82%, and superior thermal stability and fatigue properties at record high operating temperature of 400°C. These ultrahigh-temperature performances are achieved through a relatively simple method of introduction and engineering of interfaces within the capacitors, which greatly improve their high-temperature stability, relaxation behavior, and breakdown strength. Our work not only successfully fabricated capacitors with potential applications in high-temperature electric power systems and electronic technologies but also opens up a promising and general route for designing high-performance electrostatic capacitors through interface engineering.