Stiffness-tunable BaTiO3/polydimethylsiloxane (BT/PDMS) dielectric elastomer composites using dimethylsilicone oil (DMSO) as the stiffness tuner were prepared. Significant improvements in both electromechanical actuation sensitivity (0%–490%) and actuation areal strain (0%–350%) can be observed from BT/PDMS samples with DMSO, which should be mainly contributed to the physical swelling behavior. Under wide testing temperature range, the stiffness-tuned BT/PDMS composite exhibited an excellent thermal stability of both dielectric and mechanical performance. The electromechanical sensitivity of BT/PDMS composite can be effectively tuned higher than that of VHB 4910 acrylics via the addition of DMSO under in vivo temperature range (20 °C–40 °C). This simple way endows PDMS-based composites the higher performance for being applied as implantable dielectric elastomer materials. This work was financially supported by NSF of China (Grant Nos. 51425201 and 51377010), Ministry of Education of China through Doctor Project (Grant No. 20130006130002), the National Basic Research Program of China (973 Program) (Grant No. 2014CB239503), and State Key Laboratory of Electrical Insulation and Power Equipment (EIPE14204).