Reconstruction of shallow geomorphology and tectonics, based on the multiple closed system of low- temperature thermochronogical features, has received much attention and application in study of formation process of fold- thrust belt and foreland basin. The low- temperature thermochronogical age of the multiple closed system in the Longmenshan area, eastern margin of the Qinghai- Tibet plateau generally shows a decreasing trend from the foreland of the thrusting zone to interior of the plateau, and from NE to SW. The thermochronological ages of the Longmenshan fold- and- thrust belt varies greatly, distinctly larger than that inside the plateau, revealing a shallow erosion effect of enhanced fold- thrusting in the transition zone of basin and mountain during Cenozoic. Based on low- temperature thermochronological study and erosion thermal model of fold- thrust belt and foreland basin in the Longmenshan region, the study reveals that the Qinghai- Tibet plateau expands eastward at a rate of 5~10 mm/a, with an erosion rate of 0. 4~1. 0 mm/a, and a shortening rate of deformation with 0~15 mm/a. These data are consistent to features of geology and geodesy. Thermochronologic age characteristics of the W- E- trending multiple closed system in the eastern margin of the Qinghai- Tibet plateau reflect an eastward extension process of the steady plateau during the Cenozoic, i.e., the coupling process between the thrusting expansion of the Longmenshan fold- thrust belt and shallow erosion.