It is very important for underwater flextensional transducers to accurately control the prestress applied to the active element in assembling and working process, especially, for the rare-earth magnetostrictive flextensional transducer, to be investigated clearly the relationship between the working in different water depths and the compressive prestress applied to the active element,which will be conducive to employ the active element potential energy. On coordination with the author's research project, the assembly displacement of an underwater flextensional transducer, by which is applied prestress to the active element, is calculated with the finite element method, and an elementary experiment is performed with a class VII flextensional transducer. The magnitudes of compressive prestress is variable when working in different water depths, the relationship is calculated by using finite element method(FEM).The method used in this paper may be generalized to other kinds of flextensional transducer.