Abstract: The underwater broadband acoustic parametric array can produce a broadband audio beam at low frequencies with high directivity.In this paper,its theoretical self-demodulation model of generating an audio beam at low frequency is deduced firstly.Then,the directivity of its transducer is analyzed and designed,and prototype is also fabricated.The experimental result shows that the prototype can generate an audio beam at 2 kHz with excellent directivity performance,whose directivity acute angle at -3 dB is about 2.6°.Moreover,the prototype can produced an audio beam at low frequency without sidelobes due to its excellent performance in suppressing sidelobe,which is significant for solving the problems such as the multipath interference of the shallow-water communication and the long distance underwater secret communication.