Abstract: A Fish-Mouth flextensional transducer with nested double shells is presented.The hydrostatic analysis is done at first.A series of characteristic parameters which could represent the hydrostatic deformation for the air-backed flextensional transducer with special structure are summarized.The relationship between them and the structural parameters is studied in detail by finite element analysis.On the basis of the hydrostatic design,the optimization research on the electroacoustic performance of the transducer is carried out and the optimal structural parameters are determined.Then a prototype is manufactured and measured.It is shown that this projector could achieve low frequency,high power and high efficiency performance from compact size by both the simulated and experimental results.The maximum sound level reaches to 199.5 dB at the resonant frequency of 800 Hz with an electro-acoustical efficiency of 23.6%.and the effective working depth is no less than 200 m.