The Indian plate began to subduct under the Eurasian plate in the late Mesozoic. Since the Eocene, continuous continental collision has resulted in the uplift of the Tibetan Plateau with extensive volcanic activity. Geophysical and geochemical studies have suggested that the Indian plate was subducted and that volcanism originated in the deep upper mantle. However, the depth range of the subduction and volcanism and the relationship between them are still unclear. Here, to image the mantle transition zone (MTZ) structures, we deployed new seismic stations and collected as much seismic data as possible from the Tibetan Plateau. We then calculated the receiver functions and analyzed them using the common conversion point stacking method. We found that the MTZ is thickened by ~20 km under the Lhasa block, where high-velocity anomalies are obvious in the MTZ. Apparent thinning of the MTZ by ~20 km was revealed under the Qiangtang and Songpan-Ganze blocks, confirming a component of depleted mid-oceanic ridge basalt in the magmatic rocks. Our results suggest that the Indian Plate has at least subducted to the MTZ, which has further induced mantle upwelling and volcanism in the Tibetan Plateau.