Advanced dielectric film capacitors with high energy density and charge-discharge efficiency are urgently needed with ever-increasing demand of energy storage devices for electronics and power systems. Herein, polyetherimide (PEI)/boron nitride nanosheets (BNNSs) nanocomposites with homogeneous structures prepared by facile solution-processing are demonstrated. It is found that the incorporation of BNNSs enhances multiple dielectric properties of polymer matrix including DC dielectric breakdown strength and volume resistivity. Compared with pristine PEI film, the PEI/BNNSs nanocomposite films exhibit concomitant enhancements in discharged energy density and charge-discharge efficiency which is owing to the increase of breakdown strength and the decrease of dielectric loss. Additionally, excellent high-temperature energy storage capabilities also have been achieved in the PEI/BNNSs nanocomposites, e.g. 2.83 J/cm3 of discharged energy density measured at 450 MV/m and 150 oC. This composite approach paves a way of flexible polymeric dielectrics for high-temperature energy storage applications.