Flexible dielectrics with high energy density (U e) and low energy loss (U l) under elevated electric fields are especially attractive for the next-generation energy storage devices, e.g., high-pulse film capacitors. However, raising U e by introducing high dielectric constant materials generally increases U l, which is detrimental to the devices. To overcome this trade-off, a new strategy consisting of elevating electron polarization through the incorporation of 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxy (TEMPO) units and capturing electrons with stable organic radicals is proposed. A series of flexible poly (dicyclopentadiene norbornene-ended 4-hydroxy-2, 2, 6, 6-tetramethylpiperidin-N-oxy) (PNB-D x T y ) copolymers with excellent dielectric and physical properties, thermal stability, and processability, were prepared and their energy storage properties investigated. Specifically, dicyclopentadiene (DCPD) groups were designed to construct a cross-linked network, while the TEMPO groups offered a stable radical to capture electrons under the action of an electric field, which favored the enhancement of the dielectric constant, Young’s modulus, and breakdown strength of the polymers. Compared with commercially available state-of-the-art polymer dielectrics, e.g., biaxially oriented polypropylene (BOPP), PNB-D x -T y demonstrated an exceeding dielectric performance and low-cost potential. Importantly, a discharge energy density of 10.6 J/cm3 with a high efficiency of 92% at 500 MV/m was achieved. The effective strategy revealed that the isolated stable radical in the low-polarity polymer matrix suppressed the energy loss and created a new paradigm for high-energy and low-loss flexible capacitors.