Nanocomposites combining high aspect ratio nanowire fillers and a high breakdown strength polymer matrix have been actively studied for pulsed power capacitor applications. The relationship between the aspect ratio of nanowires and the dielectric constant of the composites, however, has not yet been established due to the lack of dielectric theory study, which impedes the research progress on nanowire/polymer composites for energy storage applications. In this work, a modified dielectric model based on Maxwell-Garnett approximation has been developed to quantitatively investigate the relationship between the aspect ratio of nanowires and the dielectric constant of the composites. Selecting SrTiO3 nanowires as the fillers, SrTiO3/P(VDF-CTFE) nanocomposite films were prepared using SrTiO3 nanowires with an optimized aspect ratio (∼100) by a high-speed stirring hydrothermal process. The experimental results confirm that the nanowires with the optimized aspect ratio enhance the dielectric constant and breakdown strength of the composite, thus greatly improving the energy storage performance. This work provides a universal computational approach for understanding the effect of the aspect ratio of 1D nanofillers on the composite properties, being beneficial to nanocomposite design for energy storage applications.