Herein, high-dielectric-constant titanium dioxide (TiO2) nanoparticles, modified by γ-methacryloxypropyl trimethoxy silane (referred as mTiO2) and polar dioctyl phthalate (DOP), were introduced into natural rubber (NR) to obtain high-performance dielectric elastomer composites. The incorporation of DOP sharply decreased the elastic modulus of mTiO2/NR composites by decreasing the entanglement between NR chains and weakening the enforcement effect of mTiO2 nanoparticles. At the same time, highly polar DOP improved the dielectric constant of mTiO2/NR composites and resulted in high electromechanical sensitivity (β). Finally, a large actuated strain of 25.3% has been rendered by 50 phr DOP/mTiO2/NR composite at 40 kV/mm, which is ~10 times higher than pure NR (2.6%) under the same electric field. The present study reveals that the plasticizer-enhanced flexibility of inorganic dielectric filler/polymer composites is an effective method to enhance the electromechanical performance of dielectric elastomer actuators.