Abstract: A phenomenon was found in the sea trial data of the South China Sea in autumn 2021, which showed that the phase velocity dispersion results of very low frequency (VLF) signals received at the seabed interface was discontinuity distributed in the range of 1−12 Hz. To solve this problem, a dispersion model suitable for stratified seafloor was derived. The cause of this phenomenon was discussed based on numerical simulation and sea trial data, and the physical mechanism was explained by normal mode theory. The results show that the soft-elastic sedimentary layer in the seabed leads to the discontinuity of the mode dispersion results in VLF signal. Because the modes in VLF range interacted with the shear waveguide of the sedimentary layer at some frequencies, they had the characteristics of weak excitation and strong attenuation. Modes can only propagate efficiently in frequency bands that were less affected by shear waves, which leads to the generation of modal phase velocity dispersion discontinuities in VLF signals. This dispersive discontinuity property is also affected when the parameters of the sedimentary layer are changed. The soft-elastic characteristics of the sedimentary layer should not be ignored when investigating the acoustic field characteristics of the seabed VLF in the sea trial area.