Polymer nanocomposites with high energy density have become a research hotspot in the field of dielectric materials. However, the huge compatibility contrast between nanofillers and polymers always hinders the further improvement of dielectric properties. Meanwhile, next-generation dielectrics should possess excellent thermal stability to cope with the development of high-temperature applications. Herein, a novel polymer nanocomposite based on heat-resistant poly(p-phenylene benzobisoxazole) (PBO) and high-permittivity CaCu₃Ti₄O₁₂ (CCTO) nanoparticles was prepared by the solution method. In this process, electroactive polymer poly(2-isopropenylbenzoxazole) (P(2-IBO)), which has a chemical structure similar to PBO and contains numerous permanent dipoles, is selected as the modifier to optimize the electric field behavior at the interface, and it is coated on the surface of CCTO nanoparticles via surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization for achieving controllable dielectric properties. The results demonstrate that the dielectric properties can be adjusted by changing the thickness of the P(2-IBO) shell. The breakdown strength and energy density of the modified CCTO/PBO nanocomposites are much higher than those of the unmodified CCTO/PBO nanocomposites. Under an electric field of 200 kV mm⁻¹, the maximum discharge energy density reaches 3 J cm⁻³, which is about 500% that of pure PBO (0.6 J cm⁻³). Moreover, the dielectric properties are nearly independent of the testing temperature (25 to 200 °C), indicating that this nanocomposite is an ideal candidate for high-temperature dielectric materials.