Dielectric polymers with high energy density have received widespread attention in the fields of modern electronics and power systems. Thus far, it is urgent to increase stored energy density of dielectric materials owing to the unappeasable energy density of the current commercial dielectric film caused by the inherent low dielectric constant. Herein, all-organic blended dielectric films consisting of acrylonitrile butadiene rubber (NBR) as organic fillers and poly (vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as matrix were fabricated. Moreover, thermal treatment including hot press and quenching process was utilized to modify the surface roughness of films. It is demonstrated that flattened surfaces of prepared films with thermal treatment can result in reduction of local electric field distortion and charge injection. An elevated breakdown strength of 510 MV/m, which is 113.8% of P(VDF-HFP) films, and a raised dielectric permittivity of 10.08, which is 116.5% of P(VDF-HFP) films, have been achieved in the thermal treated NBR/P(VDF-HFP) films with 2 wt % loading under room temperature conditions. The enhanced insulating strength could be resulted from the smooth surface morphology by thermal treatment and mitigatory electric field distortion. Consequently, a high energy density of 11.3 J/cm³ is obtained concurrently. In addition, numerical simulations including finite element methods and phase field calculations are calculated to explain the facilitation of insulating properties. The all-organic dielectric polymer with thermal treatment provides a feasible example for fabricating energy storage dielectrics with high breakdown strength.