High-temperature performance of energy storage dielectric polymers is desired for many electronics and electrical applications, but the trade-off between energy density and temperature stability remains fundamentally challenging. Here, we report a general material design strategy to enhance energy storage performance at high temperatures by crosslinking a polar polymer and a high glass-transition temperature polymer as a crosslinked binary blend. Such crosslinked binary polymers display a temperature-insensitive and high energy density behavior of about 6.2 ~ 8.5 J cm⁻³ up to 110 ℃, showing a significant enhancement in thermal resistant properties and consequently outperforming most of the other ferroelectric polymers. Further microstructural investigations reveal that the improved thermal stability stems from the confinement effect on conformational motion of the crosslinking network, which is evidenced by the increased rigid amorphous fraction and steady intermolecular distance of amorphous regions from temperature-dependent X-ray diffraction results. Our findings provide a general and straightforward strategy to attain temperature-stable, high-energy-density polymer-based dielectrics for energy storage capacitors.