The crust and uppermost mantle beneath the southeastern (SE) margin of Tibetan Plateau record the lateral expansion of Tibet and far-field effects from the ongoing continental collision and convergence. In order to obtain constraints on the deep structure and the eastward expansion of the plateau, we synthesized Deep Seismic Sounding (DSS) results in this region and constructed the P wave velocity models for each terrane. The principal characteristics of the crustal velocity model and tectonic significance are: (1) Crustal thickness decreases from 53 km beneath the Songpan-Ganzi terrane (SGT) to 34 km in the south direction beneath Simao terrane (SMT), and 42 km in the east direction beneath Chengdu area. The crust is dominantly felsic in upper crust, with a small percentage of mafic composition in the lower crust. (2) The crustal structure is very heterogeneous in this region. The Indochina Block is characterized by low seismic velocities in the upper crust, while the west Yangtze terrane (WYZT) and SGT show higher velocities in the upper crust. The lower crust beneath WYZT is relatively thicker than other terranes. (3) We deduce that the high velocity in the upper crust and relatively thick lower crust beneath the SGT and WYZT can be associated with magmatic processes that generated the Emeishan flood basalts which may have been triggered by rifting or mantle plume. The lateral variations of the crustal thickness in SE Tibet may be due to lower crustal flow from the Tibetan Plateau to the SE direction after blocked by the cold and rigid Yangtze craton in the east direction.