<p>The rapid development of high-power electronics and electrical transmission systems has created an urgent demand for polymer dielectrics capable of simultaneous electric field grading and thermal management. However, conventional field-grading materials (FGMs) face inherent limitations, including high filler loading requirements, limited switching field tunability and poor thermal conductivity. Here, we report a novel class of smart insulation-conductive reversible dielectrics with customizable potential barriers constructed by electrostatic self-assembly of magnetic nanoparticles onto SiC nanofibers, followed by magnetic field-induced vertical alignment in polymer matrix. This aligned architecture precisely instructs the transfer pathways for both charge and heat conduction. The barriers between magnetic nanoparticles and semiconductor fillers, flexibly controlled by the composition of magnetic nanoparticles, endow novel FGMs with prominent electric field-grading performance and ultra-high nonlinear coefficient. The introduction of deep trap states induced by magnetic nanoparticles enables precise control over a broadly tunable switching field. Notably, the novel FGMs achieve a record-high nonlinear coefficient and an exceptional thermal conductivity enhancement efficiency per 1 vol% filler loading, while maintaining excellent electrical endurance and thermal stability throughout electrical-thermal cycling tests. Given the diverse voltage levels and power densities application of novel FGMs through precise customization, these findings demonstrate significant potential for next-generation power electronics and advanced electrical insulation systems.</p>