An internal solitary wave (ISW) significantly affects acoustic propagation; however, its detailed characteristics are poorly understood. Simulation experiments of sound propagation in a shallow water environment are presented to examine the effects of the source conditions and characteristics of the ISW on transmission loss (TL). The results show that the TL decreases as the depth of the source increases and the frequency of the source decreases and that the different characteristics of the ISW are highly important for estimating sound propagation when a SONAR system is in an ISW environment. Coupled normal mode theory is further employed to analyse the variations in coupling between sound field modes in an ISW environment. Further analysis reveals that the magnitude of the TL is affected by the direction and fluctuation of energy propagation between different modes, and in different ISW environments under the deep and low-frequency source conditions, the sound field energy is mainly in lower-order modes.