In this paper, the polydimethylsiloxane (PDMS), carbon black (C), and barium titanate (BTO) nanoparticles prepared by the hydrothermal method were composited and spin-coated to obtain a BTO/PDMS/C flexible piezoelectric composite film. The results show that the prepared BTO nanoparticles have a uniform size with diameter of about (58±10) nm. When the content of C is 12%, the BTO/PDMS/C flexible composite film has the best output performance, and the output voltage and current are 11.14 V and 6.23 μA, respectively, and the output power of 8.45 μW can be obtained at a load resistance of 5 MΩ. After 4 000 cycles of testing, the prepared BTO/PDMS/C flexible piezoelectric composite film has a stable piezoelectric output, which is a good choice for manufacturing wearable electronic devices and monitoring components.