A high sensitive and low cost hydrophone based on resonant air cavity is proposed in this paper. The hydrophone is formed by a sealed hollow spherical shell/cavity with a microphone inside to sense the sound coupled from the water outside. The frequency response of the hydrophone is predicted by finite element simulation, and it is confirmed that the characteristic peak of the frequency response curve depends on the resonant mode of the air cavity and is independent of the shell resonance. Therefore, the resonant frequency of the hydrophone only depends on the size of the internal air cavity. The experimental results demonstrate that the frequency response of the hydrophone is consistent with the simulation results, and the sensitivity is improved compared with the conventional piezoelectric hydrophone. The receiving sensitivity at the resonant frequency is 10 times higher than that of the conventional piezoelectric hydrophone.