Abstract: To overcome the limitations of traditional piezoelectric ceramics in terms of elastic compliance constant, electromechanical coupling coefficient, and energy density, thereby enabling compact, broadband, high-power underwater acoustic transducers, this study leverages the material advantages of relaxor ferroelectric single crystal. A longitudinal vibration transducer driven by the transverse extensional mode (32-mode) of a single crystal is investigated, establishing its electro-mechano-acoustic equivalent circuit model. This model reveals the optimal matching relationship between the transducer’s effective electromechanical coupling coefficient and mechanical quality factor, demonstrating that bandwidth broadening can be achieved by regulating the mechanical quality factor. Furthermore, the underlying mechanism governing the transverse resonance orthogonal beam (TROB) mode of the single crystal is elucidated. Through the optimization of structural parameters for both the front mass and the single-crystal oscillator, a multi-mode, coupled, ultra-broadband single-crystal transducer was realized, integrating the longitudinal vibration mode, TROB mode, and the bending mode of the front mass. A prototype was fabricated to validate the bandwidth extension mechanism. The prototype exhibits a transmitting voltage response (TVR) that varies by 3.2 dB over the 16.5~73.0 kHz frequency band. Measured results show excellent agreement with theoretical predictions, confirming the effectiveness of the bandwidth extension approach.