The large-scale morphology of the eastern margin of the Tibetan plateau can be divided into three zones, the Tibetan Plateau, Longmenshan and Sichuan Basin. To assess the large-scale erosional thickness and erosional rate of the region, we use digital elevation models (DEM) and fission-track thermorchronology to calculate the erosional thickness across the margin. We calculated the erosional rate of Longmenshan in the last 3. 6 Myr (ESR) , and the results indicate that the erosional thickness of Longmenshan is 1. 91-2. 16 km, and the erosional rate of Longmenshan is 0. 53-0. 60 mm/a. Based on results of simulation by flexural deflection, we inferred that the mountain building model of Longmenshan would have been constrained to both erosionally-driven uplift and tectonic shortening-driven uplift. Before 3. 6 Ma, the uplift of Longmenshan is driven by tectonic shortening related to the India-Asia collision, and after 3. 6 Ma the uplift of Longmenshan is driven by erosional unloading.