Abstract: A new high sensitivity piezoelectric ceramic hydrophone employing the fluid cavity coupling modal is raised.This fluid cavity coupling hydrophone adopts a radial polarized piezoelectric ceramic tube as its sensitive material who uses only the inner side to receive sound pressure meanwhile masks all the other sides. The vibration characteristic of the piezoelectric ceramic tube is analyzed in detail by the Energy method. A comparison of receiving performance is made between the new type and traditional type transducer by FEM method. Prototypes are developed subsequently. The measured results show that the fluid cavity coupling hydrophone has significant promotion in the receiving sensitivity comparing to the outer side adopted type with the same piezoelectric ceramic tube in the frequency range of 1-8 kHz.At the fluid cavity resonance frequency the promotion attains beyond 10 dB, and the receiving sensitivity of the new type hydrophone achieves-181 dB. This research shows that the fluid cavity coupling modal can promote the receiving sensitivity of the hydrophone markedly in a considerable frequency range.