Dielectric capacitors are widely used in modern electronic systems and power systems because of their advantages of fast charge discharge speed and high-power density. Nowadays, the new products related to renewable energy, such as hybrid electric vehicles, grid connected photovoltaic power generation and wind turbines, downhole oil, gas exploration, etc., put forward higher requirements for the energy storage capabilities of dielectric capacitors in elevated-temperature. In this review, the research progress of the polymer-based dielectrics for high-temperature capacitor energy storage in recent years is systematically reviewed to offer benefits for further study. Firstly, the physical mechanism of energy storage of dielectric materials is introduced, and several conduction mechanisms of dielectric materials are summarized and analyzed; then, several strategies to improve the high-temperature energy storage performance of polymer dielectrics are presented, including the nanocomposite modification and design of layer-structured polymer composites, and the molecular structure design and chemical crosslinking treatment of dielectric polymer. Finally the scientific and technological problems in the application of dielectric polymer and their nanocomposites for high-temperature capacitor energy storage are discussed, and a possible research direction in the future is prospected.