AbstractPolymer dielectrics capable of operating stably at high temperatures (>150 °C) are urgently in great demand to catch up with the booming electric power systems. Enhancing the heat resistance of polymers is typically achieved through thermal crosslinking with the use of crosslinking agents. Unfortunately, the conventional thermal crosslinking faces challenges in terms of commercialization due to its complex fabrication, inevitable introduction of small molecule impurities, and significant time and energy consumption. Here, a convenient, impurity‐free, time and energy efficient photo‐crosslinking method to create wide bandgap heterostructures within a dielectric polymer matrix, is presented. Remarkably, this approach achieves synergistically enhanced heat resistance and electrical insulation. Surprisingly, the crosslinked polymer exhibits an unprecedented Weibull characteristic breakdown strength of 1057 and 810 MV m⁻¹ at room temperature and at 150 °C, respectively, which corresponds to superior energy storage densities of 14.28 and 5.55 J cm⁻³ with a charge–discharge efficiency of 90%, respectively. Furthermore, the polymer demonstrates an excellent electrical breakdown self‐healing character. These remarkable achievements, combined with the convenient and cost‐effective fabrication process, highlight the potential of cinnamate photo‐crosslinking in advancing high‐performance polymer dielectrics.