In this paper, a sound field model of elastically coupled normal mode suitable for complex eigenvalues is established. The normalization and coupling coefficient expression of elastic normal mode are given when the leaky mode is included. The coupling coefficients satisfy the conservation of sound energy flow. This model is used to analyze the coherent coupling characteristics of the normal mode of the sound field under the condition of the inclined elastic seabed. It is found that when the leaky mode is considered, the normal mode coupling will not only cause the amplitude of the normal mode to change, but also bring additional phase shift. The simulation calculation shows that under the condition of inclined elastic seabed, the acoustic propagation loss obtained by considering the effect of the additional phase shift of the leaky mode coupling is closer to the calculation result obtained by using the finite element commercial software, and that the mode coupling greatly enhances the amplitude of interface wave. In addition, this paper also analyzes the influence of changes in marine environmental parameters on sound transmission loss.