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The enhancement of the energy storage property of capacitor films at elevated temperature has been considered a critical area of research owing to the essential requirements of capacitor applications. In the present study, a ternary composite system with an improved energy storage property is reported. The ternary composite was composed of PP-graft-AA chains (acrylic acid grafting) and a PP/nano-ZrO2 matrix and was biaxially oriented into a defect-free film with a thickness of 10 μm. The obtained film (BO(PP/AA/Zr)) exhibited an energy storage density of 7.9 J cm−3 at room temperature and maintained a considerably high value of 3.9 J cm−3 at 120 °C. The characterization of charge carrier transportation indicated that the effective biaxial orientation and the introduction of the polar grafted functional group and nanofillers increased the activation energy of molecular chain segment motion and deepened electron trap energy levels, which limited the transportation of charge carriers and significantly enhanced the breakdown strength and energy storage property. Moreover, the designed graft-modified chains (acrylic acid grafted) modified the compatibility between the polymer and inorganic nanofiller, thereby improving the processibility of the defect-free thin film. The results of dynamic mechanical analysis and 2D micro-infrared spectra indicated that the graft-modified chains offset the increased elastic rigidity caused by nanoparticles and facilitated the interaction between inorganic nano-ZrO2 and polymer chains. The study provides a potential solution to the challenge that nanocomposites face during the commercial production of defect-free biaxially oriented films. [ABSTRACT FROM AUTHOR]