Flexible polymer dielectrics for capacitive energy storage that can function well at elevated temperatures are increasingly in demand for continuously advancing and miniaturizing electrical devices. However, traditional high-resistance polymer dielectrics composed of aromatic backbones have a compromised band gap (Eg) and hence suffer from low breakdown strength and a huge loss at high temperatures. Here, based on the density functional theory (DFT) calculations, rigid and non-coplanar alicyclic segments are introduced into the polyimide backbone to overcome the incompatibility of a high glass transition temperature (Tg) and large Eg. Thanks to the large optical Eg (∼4.6 eV) and high Tg (∼277 °C), the all-alicyclic polyimide at 200 °C delivers a maximum discharge energy density (Ue) of 5.01 J cm-3 with a charge-discharge efficiency (η) of 78.1% at 600 MV m-1, and a record Ue of 2.55 J cm-3 at η = 90%, which is 10-fold larger than that of the state-of-art commercial polyetherimides (PEIs). In addition, compared with aromatic polyimides, the all-alicyclic polyimide possesses a better self-clearing characteristic due to a smaller ratio of carbon to hydrogen and oxygen, which facilitates its long-term reliability in practical applications.