Abstract: With the rapid development of underwater acoustic signal processing technique, more and more information needs to be obtained from transducers, and the working frequency band of underwater acoustic transducers ought to be broader. A broadband underwater Tonpilz transducer was designed by using the coupling of transducer′s longitudinal vibration mode and the bending vibration mode of front mass to widen its frequency band. The electro-acoustic characteristics of the transducer was simulated with finite element method. The prototype transducers were made based on the results of simulations and tested. Experimental results show that the broadband characteristics of the Tonpilz transducer agree well with the simulation results. Its maximum transmitting voltage response level is 139 dB, -3dB bandwidth is beyond one octave, and low frequency is below 15kHz. Thus the designed transducer has the advantages of low frequency, broadband and high response level.