This work proposes a new lateral extrusion dynamic model based on geological evidence, mathematical modeling and 3D discrete element simulation to explain the southeastward thrusting during the closure of the Songpan-Ganze basin. The Late Triassic NE-SW compression caused by the northward movement of the Qiangtang Block and the resulting differential shortening within the wedge-shaped Songpan-Ganze terrane produced southeastward topographic gradient. The thick sedimentary pile, decoupled from the subducting basement by the low-strength decollement, was driven by gravity and laterally extruded, leading to the southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. The mathematical modeling provides detailed description of the differential shortening occurred within the Songpan-Ganze terrane. The 3D discrete element simulation, which is first time used for the geological study of the Longmenshan area, clearly reproduces the dynamical process of the Longmenshan southeastward thrusting and well predicts the Xiaojin Arcuate Zone. Both mathematical modeling and discrete element simulation verify the new lateral extrusion dynamics and reveal that two key factors facilitate the lateral extrusion and foreland thrusting, which are the wedge-shaped geometry that produces differential shortening and lateral topographic gradient, and the low-strength decollement that decouples the extruded sedimentary pile from the basement. The lateral foreland thrust belt, which has specializations in its tectonic location and dynamic behavior, is a new kind of foreland thrust belt after pro-foreland thrust belt and retro-foreland thrust belt, and provides a new insight into the tectonic evolution of collisional orogens.