There have been many reports on observations of naturally occurring internal solitary wavetrains (solitons) in the coast zone, especially in the summer. The mechanism for the generation of these nonlinear internal waves has been investigated in the geophysics and fluid mechanics community. Little attention, however, has been focused on their effect on sound propagation with the exception of the work of Baxter and Orr, which was based on ray theory [L. Baxter and M. H. Orr, J. Acoust. Soc. Am. 71, 61–66 (1982)]. In this paper, the parabolic equation (PE) model (IFD or PAREQ code) is used to numerically simulate the effect of internal solitons on low‐frequency sound propagation in the coast zone. The results show that sound transmission loss is sensitive to soliton parameters (such as wavelength, amplitude, etc.) and wave packet parameters (position, number, and propagation direction). The results can be used to explain, at least partly, an interesting experiment phenomena: The frequency response of shallow‐water sound propagation under the condition of a thermocline during the summer is often a strong function of space and time, and sound propagation over certain frequency ranges has an abnormal attenuation that cannot be explained by using conventional models of sound propagation. [Work supported by ONR.]