Abstract: In order to realize a low-frequency and small-size underwater acoustic source, an underwater acoustic transducer has been designed by using NiMnGa alloy, which has the advantages of large strain, rapid response and high energy density. Based on the deformation principle of NiMnGa alloy, a physical model of a NiMnGa longitudinal transducer is established, and the equivalent circuit of the transducer is derived. Ａmulti-physics coupling model of electromagnetic, mechanical and acoustic fields is established by using the finite element method to predict the underwater acoustic performance of the transducer. A small-scale prototype of the NiMnGa longitudinal transducer is fabricated and its sound source level is tested in water within the frequency range from 500 Hz to 800 Hz. Experimental results demonstrate that the NiMnGa transducer with an 8 mm diameter of the radiation surface, achieves a maximum sound source level of 115.5 dB at 700 Hz.