The surface wave method is a promising non-invasive test technique, which is cost-saving, convenient, and environmentally friendly. However, some fundamental issues still require further improvements. For the relationship between the penetration depth and wavelength of the surface wave, some researchers adopted an assumption that the penetration depth is within the range of 0.5 to 1.0 wavelength. In this study, to evaluate this assumption quantitatively, a numerical model for two layers of soil was established. A series of dispersion curves were obtained by varying the thickness of the upper layer. According to the results, when the ratio between the thickness with the wavelength equals a constant value, the dispersion phenomenon happens. The results of a parametric analysis on the minimum dispersive wavelength in different thicknesses show that this constant ratio is close to 1.67 which is much larger than the previous assumption. This result implies that the dispersion curve can be used to infer a deeper soil stiffness profile.