It is of great significance to develop piezoelectric composites with mechanical flexibility and high electromechanical coupling characteristics for the collection and conversion of mechanical energy. In this work, a barium titanate (BTO) based piezoelectric composite was prepared by hydrothermal synthesis and high-temperature carbonization with the assistance of cellulose nanofibers (CNF), graphene oxide (GO) and carboxylated carbon nanotubes (C-CNT). C-CNT, GO and CNF can provide growth sites for BTO, but compared with C-CNT, CNF and GO have stronger interaction with BTO, which can improve the compatibility between BTO and CNF/polyvinylidene fluoride (PVDF) matrix. The tensile strength of the composite membrane prepared by GO-CNF-BTO treated at 1100 °C can reach 23.36 ± 8.30 MPa. After further hot pressing and polarization treatment, its longitudinal piezoelectric coefficient (d33) reaches 9.8 ± 2.6 pC·N−1. Correspondingly, the open-circuit voltage (VOC) and short-circuit current (ISC) increases to 11.64 V and 824 nA, respectively. In addition, the assembled piezoelectric generator (PEG) can charge for small commercial capacitors, as well as light small bulbs and electronic screens. It also has good piezoelectric output stability and sensing performance, which can effectively convert the mechanical energy from human motion into electrical energy. This work provides a reference for the performance improvement of flexible piezoelectric devices.