Abstract: A fast algorithm of sound propagation in three dimensional underwater environments is presented in this paper. On base of the generalized phase integral (WKBZ) theory and the Beam-Displacement Ray-Mode (BDRM) theory, the Coupled Mode Parabolic Equation (CMPE) theory of sound propagation in range dependent underwater environment is extended for three dimensional (3D) problems. The CMPE3D solution is expressed in terms of the normal modes in vertical direction and the coupled mode coefficients in horizontal directions. By using the WKBZ theory and the BDRM theory, the normal mode analysis can be processed efficiently. A PE-based algorithm, which was implemented in model FOR3D by Lee D. et al. ("Numerical Ocean Acoustic Propagation in Three Dimensions", World Scientific, Singapore, 1995) is also adopted to solve the coupled mode coefficients. Numerical simulations indicate that the efficiency has been greatly improved by using CMPE3D instead of the PE approximations.