Abstract: An underwater acoustic transducer with dual-Helmholtz resonator is presented.The acoustic output of flexural disc transducer below its resonant frequency can be amplified by Helmholtz resonators,the "∞" beam pattern can be obtained due to in-phase radiation of two Helmholtz resonators.Thus,the transducer has the characteristic of low frequency directional transmitting which is difficult for low frequency,small size transducers.The principle of achieving directivity for the low frequency transducer is described.Finite element model is modeled by ANSYS software for studying its acoustic performance.The influence of key structural parameters such as the length of cavity,the thickness of metal disc in flexural disc transducer and the thickness of simply supported structure on Helmholtz resonant frequency have been investigated.The transmitting voltage response and directivity are solved.The prototype is fabricated based on simulation dimension result,transmitting voltage response and directivity are tested in anechoic tank.Test results revealed that the transducer has "∞" vertical directivity at Helmholtz resonant frequency,the beam pattern and Helmholtz resonant frequency are in good agreement with simulation results.The transducer presented in this paper can provide technical support for low frequency directional transmitting with small size.