The India-Eurasia collision over the past 50~65 million years is undoubtedly the most spectacular and youngest tectonic event on Earth, which made the Tibetan plateau uplift by >4 km and its crust thickened to twice normal thickness (~70 km).Field observations and satellite geodesy indicate that the uplift of the Tibetan plateau has been accompanied of a small crustal shortening from the center to the southeastern margin since about 4 million years ago. The Sichuan region located at the eastern margin of Tibet is characterized by steep topographic relief, where the elevation increases from 1000 m to 4000 m over only 50~100 km distances across the Longmenshan fault. The mechanism of intense deformation in the crust and upper mantle and the dynamic model of this region remain one of debate issues. The lower-crust flow model is one of several competing models to interpret the growth and expansion of the southeastern Tibetan plateau, in which the strength of the lower crust is several orders of magnitude less than that of the upper crust, so that the mass of lower crust is flowing from central Tibet to Yunnan.The model seems to be feasible and is generally accepted by many geoscientists because it provides a satisfactory explanation for the topography variations and the lack of substantial shortening of the upper crust in the eastern margin of the Tibet plateau. Nonetheless, this model, despite its potential for a reasonable explanation of the regional geodynamics, is still the subject of lively debate due to the lack of conclusive evidence. In this study, we used teleseismic data recorded by 49 permanent broadband stations deployed in the Sichuan region to obtain the P receiver functions and determined the crustal thickness and Poisson's ratio, permit to build an initial model for inversion. Based on the linearized inversion algorithm, we introduced a two-step inversion technique, in which the S-velocity structure beneath stations is obtained by fitting the low-frequency and high-frequency receiver functions, respectively. Synthetic experiments indicate that this technique can effectively reduce the dependence of inversion on initial models. In order to obtain optimal solutions, the Bootstrap resamping technique is applied to estimate the uncertainty in solutions. Our results show that the crustal thickness beneath the Sichuan basin is 40~46 km, increasing from 46~52 km beneath the northern Songpan-Garzê fold to 50~60 km beneath the southern Songpang-Garzê fold. In general, the crustal thickness increases by 10~15 km from the Sichuan basin westward across the Longmenshan fault. In the Sichuan basin and adjacent regions, the crustal Poisson's ratio is 0.25~0.32, exhibiting a regional distribution. For example, the high Poisson's ratio (0.28~0.30) appears along the Longmenshan fault and the Anninghe-Xiaojiang fault, respectively. The S-velocity structure shows a low-velocity zone in mid/low crust from the central Tibet that may be resisted by the rigid Sichuan basin, so that it redirects to extend along the Longmenshan fault, and results in the steep topography and the thickening of lower crust beneath the fault due to the accumulation of low-velocity zone.