High-temperature-resistant dielectric films, the heart of energy storage components in film capacitors, are key elements to ensure that the capacitors operate properly in harsh environments. Herein, a kind of flexible thermal-resistant poly(arylene ether nitrile) (TR-PEN) dielectric film with controllable high-temperature resistance is fabricated through post self-crosslinking of PEN at different temperatures for different times. The dielectric films can present extremely high thermal stability (T-g > 370 degrees C), a long service life at 300 degrees C (4.5 x 10(4) min, 95 wt % of the residual weight), and a low temperature coefficient of dielectric constant from 50 to 300 degrees C (<5.x 10(-4) degrees C-1). The retention of the energy density of TR-PEN350 at 300 degrees C is higher than 80% compared with that at room temperature. These results indicate that the TR-PEN films can be used over a long term as capacitor films at temperatures up to 300 degrees C. Besides, the simple and controllable fabrication technique for the TR-PEN can be easily industrialized.