Since the Late Cenozoic, the central-southern Longmen Shan (LMS) region is characterized by a two-décollement thrust-fold system with a localized upper evaporite weaker layer just beneath the Sichuan Basin and the lower stronger décollement beneath the LMS. To date, the role of upper décollement and erosion in shaping the structural deformation of the LMS remains unknown. The investigation of this question may shed lights on the assessment of the structural evolution and the further discrimination of mountain building mechanism. In this work, a 2D finite difference method is applied with regards to the variations of viscosity/friction angle, and lengths of upper décollement under different erosion rates. Two types of numerical models are setup, with purely lower-upper frictional (FF) décollements and lower-frictional upper-viscous (FV) décollements. Tectonic evolution with FF models exhibits large similarity with that of FV models. Meanwhile, our results underline the presence of in-sequence fault propagation procedures during most of the stages. The development of the boundary ramp, the Range Frontal Blind fault, is strongly induced by a prerequisite with upper décollement of necessary length. The activation of Beichuan/Pengguan faults or blind ramp piercing to the surface is extremely mechanically sensitive to the viscosity of the salt décollement, erosion rate, and the length of the upper décollement. Out-of-sequence thrusting, shallow and possible basal accretion processes beneath the Sichuan Basin are found when deformation transfers from the thick-skinned LMS to the thin-skinned Sichuan Basin at the very late stage, supporting upper crustal shortening dominates the mountain building mechanism.