There exist evident differences in crustal structure and thickness among various terranes in Qinghai-Tibet Plateau. The crustal structure histograms of these terranes are presented. Studies show that it is the tectonic superposition of the crust caused by the collision of terranes that results in the development of the low-velocity layers in the crust of most terranes. The shallower crustal layer with felsic-intermediate composition is considered to be the lower half of the crust, with the original composition preserved. The materials of the low-velocity layers have not been subjected to partial melting because the temperature remains lower than the melting solidus.