Among various types of energy storage devices, dielectric capacitors show unique advantages such as a high power density and a fast charge/discharge rate. The dielectric material is a key material that affects the energy density of dielectric capacitors. Dielectric capacitors complete the storage and release of energy precisely through the polarization behavior of the dielectrics. To achieve a goal of dielectric capacitors to gradually move towards lightweight, small-sized, and integration, it is necessary to further improve their energy density. Dielectric constant and breakdown field strength are two important factors affecting the energy density of dielectrics. However, improving the dielectric constant and breakdown field strength is still a great challenge.As different types of dielectrics are thoroughly investigated, polymer-based composite dielectrics show a great potential for energy storage because they have a high dielectric constant and a low dielectric loss of inorganic dielectrics, and a unique flexibility and a superior breakdown resistance of polymers. However, there are some problems with polymer-based composite dielectrics that restrict the further improvement of their energy density. It is necessary for the improvement of the dielectric constant of polymer-based composite dielectrics to add a high content of dielectric constant fillers. However, a high content of fillers is prone to agglomeration. Large polarization differences between fillers and polymer matrix result in an interfacial compatibility issue, leading to an early breakdown in composite dielectrics. It is thus difficult to effectively improve the energy density.The structural design of the fillers and the macrostructural modulation of the dielectric composites are expected to optimize the dielectric properties. The core-shell/coaxial structure of fillers can be designed to alleviate the problem of decreasing breakdown field strength due to the large difference in dielectric properties between polymers and fillers. For instance, cladding the filler surface with a material of medium dielectric constant can improve the electric field distortion. Recent studies reported the introduction of two-dimensional fillers with large aspect ratios into polymer matrices. Two-dimensional fillers have a smaller specific surface energy, which makes it easy to achieve a uniform dispersion in the polymer matrix. Two-dimensional fillers also promote the scattering of carriers, which delays the breakdown and improves the breakdown performance of composite dielectrics.For the negative correlation between breakdown field strength and dielectric constant, many macro-regulations of polymer-based composite dielectrics are also investigated based on the development of polymer-based composite dielectric macro-control measures. Sandwich structures and composite dielectric structures with up to tens of layers are developed. The dielectric layers with different filler contents are designed to regulate the spatial distribution of the electric field, restricting the local weak electric field to a low filler content layer to inhibit the breakdown process of the dielectric. A high filler content layer enhances the dielectric constant of the composite dielectric. In addition, the use of layered films with a combination of linear and nonlinear dielectrics is expected to achieve a simultaneous increase in energy density and efficiency.As capacitors are widely used in high-power energy storage scenarios such as aerospace and electric vehicles, higher requirements are placed on the thermal stability and dielectric properties of dielectrics at high temperatures. The high conduction loss of polymer-based dielectrics at high temperatures severely weakens their energy storage performance. A major strategy that can reduce the conductivity loss at high temperatures is to introduce wide bandgap inorganic fillers such as boron nitride nanosheets and alumina nanosheets into the polymer matrix. Another strategy is to apply wide bandgap inorganic coatings on the film surface to increase the potential barrier at the electrode-dielectric interface and hinder charge injection. An idea to enhance the high-temperature energy storage performance of composite dielectrics is the structural modification of polymer molecular chains such as grafting or cross-linking treatments, which introduces charge-trapping sites and hinders charge transport, thus suppressing the conductivity loss at high temperatures.Summary and prospects Micro-structural and macro-structural designs both can enhance the energy density of polymer-based dielectrics. However, there are still some issues to be further addressed, namely, the interfacial interaction mechanism between filler and polymer and the breakdown mechanism of the layered structure. The carrier dissipation and heat accumulation at high temperatures are also aspects to be explored. In summary, the investigation of the interfacial bonding and breakdown mechanism of polymer-based composite dielectrics and the enhancement of the high-temperature energy storage performance of composite dielectrics will promote the dielectric capacitors to achieve broader application prospects.