In this paper, an innovative strategy is proposed to enhance the high-temperature dielectric properties of polypropylene through the synergistic modification of difurfuryl disulfide (DFDS) functionalization and long-chain branching. DFDS was grafted onto the PP molecular chain by free radical reaction, which utilized the synergistic effect of its bifurfuryl moiety and disulfide bond to construct a deep trap, and at the same time enhanced the thermal stability of the molecular chain. Experiments showed that the successful grafting of DFDS increased the crystallization temperature to 120 °C, the melting point to 168.7 °C, and the crystallinity and crystal regularity were significantly optimized. The dielectric test showed that the dielectric constant of the modified sample with 0.5 wt% DFDS was improved compared with pure PP at 100 °C, and the dielectric loss was lower than that of pure PP in all frequency bands; the conductivity was reduced to 1 × 10−12 S/m at 125 °C, which was 96% lower than that of pure PP; the breakdown strength reached 28–34 kV/mm at 100 °C, and the energy storage density remained 2.42% at 130 °C. The dielectric strength of the sample with 0.5 wt% DFDS was also increased to 1 × 10−12 S/m at 125 °C, which was 96% lower than that of pure PP. The breakdown strength reaches 28–34 kV/mm at 100 °C, and the energy storage density remains at 2.42 J/cm3 at 130 °C, a 66% increase over pure PP. This study provides a new idea for the molecular design of high-temperature dielectric films.