Forward modeling was carried out on two cavern physical models with different scale and different fillings. After prestack migration, the imaging width of caves was analyzed. On the prestack migration section, the imaging width of caves is obtained by extracting horizontal trace amplitude of the string. The result indicates that different-amplitude imaging width is sensitive to the actual cave width. The analysis on the relationship between half-amplitude imaging width and actual width reveals that the half-amplitude imaging width has amplification effect on the actual width, and along with the increasing of the actual cave width, the magnification coefficient attenuates rapidly till verges to one.