In this study, three-dimensional (3D) braided composites are integrated into the substructure of the energy harvester and a 3D braided piezoelectric composite energy harvester (BPCEH) with good performance is proposed. A theoretical model of the 3D BPCEH under the force–electricity–thermal coupling is established, and the output response, which is affected by excitation frequency, load resistance, external excitation and temperature, is simulated. The results illustrate that the braided elastic layer can greatly enhance the mechanical performance of the 3D BPCEH. With the increase in temperature and braided angle, the inherent frequency of the 3D BPCEH migrates into the low-frequency direction and can make the inherent frequency of the 3D BPCEH closer to environmental frequencies.