Abstract: With the rapid development of offshore wind power, electric drive vehicle and oil and gas exploration, film capacitors are required to operate at higher temperature. However, when the temperature reaches 120℃, the charge-discharge efficiency of the most widely used biaxially oriented polypropylene (BOPP) decreases sharply, which fails to meet the demand of applications. In this paper, biaxially oriented dielectric films of styrene-grafted polypropylene (PP-g-St) are prepared. It is found that the energy density of PP-g-St is increased from 0.23 J/cm³ to 1.67 J/cm³ (grafting efficiency is 90%) at 120℃. At the same time, the breakdown field is greatly enhanced and the high temperature leakage current is effectively suppressed, which can contribute to the deep traps introduced by grafting significantly hindering the charge transportation. After stretching, the self-polymerized phase has good compatibility with the polypropylene matrix. This study provides a potential technical route for large-scale production of polypropylene-based capacitor materials for high temperature and high field applications.