Abstract: An idea of designing broadband Janus transducer with composite cavities is presented in this paper. Based on the Janus-Helmholtz transducer, the ring resonant mode of Janus-hammer bell is added, and the working bandwidth of the transducer is effectively widened by mutual coupling of three transducer working modes. Moreover, the free-flooded design of whole transducer is adopted to achieve internal and external hydrostatic pressure balance, which is suitable for working in deep water. The working principle of the composite resonance cavity is introduced at first, and then an approximate estimation formula of the resonance frequency of ring mode is given. The validity of the approximate formula is proved by comparison with the results of the finite element analysis. The structure of the transducer is optimized by the finite element method, and an experimental prototype is prepared according to the optimization results. The final test results are in good agreement with the simulation results.