The authors carried out forward modeling on dispersion curves of Rayleigh wave in three-layer media and then analyzed characteristics of dispersion curves. It is proved that the dispersion curves never intersect with each other, thus correcting the false opinions held by some scholars. When a low-velocity layer exists, the fundamental guided curve will become complex, some guided waves will assume intersection trend, and some guided waves show phenomenon of truncation in certain conditions. According to the analyzing results, the forming mechanism of the zigzag dispersion curves has also been explained.