In this paper, reduced graphene oxide (rGO), decorated with immobilized carbon nanotubes (CNTs), was introduced into a thermoplastic polyurethane (TPU) matrix to obtain a conductive nanocomposite with 2 wt% nanofillers content. Hot-pressed composite sheets were then biaxially stretched into films to efficiently fabricate flexible strain sensors. The biaxial stretching process was shown to promote secondary dispersion and parallel alignment of the nanofillers, resulting in the sensors exhibited significantly higher sensitivity (GF = 150 for the rGO/TPU_4×4 sensor with a stretching ratio of 4 × 4 relative to GF = 3.5 for the rGO/TPU_1×1 sensor at 30% strain). Furthermore, due to the synergy of the CNTs and rGO nanoparticles, the rGO/CNT/TPU_4×4 sensor had a lower resistivity (5.62 × 10⁴ Ω·m), wider monitoring range (0.3 ∼ 400% strain), and higher stability compared with the rGO/TPU_4×4 sensor. The ability of the sensor to recognize body movements and physiological activities was also shown.