Abstract: The authors studied variation characteristics of incorporative dispersion curves with thickness and depth of a low-velocity mid-layer, and analyzed causes of zigzag dispersion. Multi-mode dispersion curves and their vertical displacements at free surface of Rayleigh waves were calculated by the transfer matrix method. Then, the authors obtained incorporative dispersion curves by the maximal amplitude method and the weighted displacement method respectively, and knew variation characteristics of incorporative dispersion curves with thickness and depth of a low-velocity mid-layer in media. According to the modeling results, we know that the basic characteristics of incorporative dispersion curves calculated by above two methods are similar, and the number of zigzag phenomena increases with thickness and decreases with depth of the low-velocity mid-layer. A zigzag phenomenon obtained by the maximal amplitude method is due to mode jumping. However, zigzag phenomena obtained by the weighted displacement method are due to mode jumping from one mode to multi-mode.