The mode coupling and energy transfer are studied by considering the influences of variation in topography on sound energy transmission and structures of interference in a range-dependent waveguide. A larger level-stepped coupled mode model and a three-dimensional coupled mode model for the wedge bottom are obtained such that the mode coupling and energy transfer may be analyzed efficiently and rapidly. According to the coupled mode models, the transfer of energy is expounded for the forward pressure field in the waveguide with varying topography. Meanwhile, the mechanism is explained by the ray-mode theory for variation of energy distribution caused by variation of topography. Numerical simulations show that the coupling between normal modes and the energy transfer may occur remarkably when the imaginary parts of eigenvalues take on a huge modification, and the propagation direction of sound field will be changed to the increasing direction of sea depth due to variation of topography. In the energy transfer and the modification of propagation direction, the energy of sound field tends to remain in the waveguide, rather than to leak to the seafloor. Meanwhile, the energy distribution will be affected by the compression or sparseness so that interference structures such as ellipse, will be produced.