A Fe₂O₃−Bi₂MoO₆ heterojunction was synthesized via a hydrothermal method. Scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, powder X-ray diffraction, Fourier transform infrared spectroscopy and ultra-violet−visible near-infrared spectrometry were performed to measure the structures, morphologies and optical properties of the as-prepared samples. The various factors that affected the piezocatalytic property of composite catalyst were studied. The highest rhodamine B degradation rate of 96.6% was attained on the 3% Fe₂O₃−Bi₂MoO₆ composite catalyst under 60 min of ultrasonic vibration. The good piezocatalytic activity was ascribed to the formation of a hierarchical flower-shaped microsphere structure and the heterostructure between Fe₂O₃ and Bi₂MoO₆, which effectively separated the ultrasound-induced electron–hole pairs and suppressed their recombination. Furthermore, a potential piezoelectric catalytic dye degradation mechanism of the Fe₂O₃−Bi₂MoO₆ catalyst was proposed based on the band potential and quenching effect of radical scavengers. The results demonstrated the potential of using Fe₂O₃−Bi₂MoO₆ nanocomposites in piezocatalytic applications.