Abstract: A coupling algorithm based on the FEM and wideband fast multipole boundary element method (FMBEM) is proposed for the simulation of the radiating and scattering problems of underwater structures. The Burton-Miller formulation is used to solve the non-uniqueness problem when using a single Helmholtz boundary integral equation for exterior boundary-value problems. The iterative solver GMRES and the fast multipole method are applied to accelerate the solution of the linear system of equations. For the problems of the low computational efficiency of the original fast multipole algorithm (FMM) at high frequencies and the numerical instability of the diagonal FMM at low frequencies, a new wideband FMM formed by combining these two methods is applied to overcome the problems. A set of optimal parameters for the wideband FMBEM is obtained by observing the performances of the wideband FMM algorithm in terms of computing time and memory usage. A numerical example is presented to demonstrate the efficiency and validity of the proposed algorithm.