Abstract: The co-seismic slip sense of the 2008 Wenchuan earthquake (Mw 7.9) has demonstrated the present east-west (E-W) crustal shortening and oblique thrusting across the Longmen Mountain. However, the oblique thrusting and its initial time remain poorly investigated. By using 3-D seismic reflection data, field investigation, and published dating results, the authors investigated the E-W crustal shortening in the Dayi Fault System (DYFS), and determined tectonic stress field changes during the Late Cenozoic. Two-period tectonic deformations were found in the DYFS, which correspond to the NE-and NS-trending structures, respectively. The activities of the DYFS and Qiongxi Fault System reflect a change in the direction of the tectonic stress, from NW-SE during the Oligocene and Early Pliocene to E-W from the Late Pliocene to Holocene. The 120km NS-trending structures in the southern Longmen Mountain range front as well as the Wenchuan earthquake co-seismic ruptures are assumed to reflect the active, E-W crustal shortening in Longmen Mountain, eastern Tibet plateau. The results obtained by the authors provide a new perspective for the uplift mechanisms of eastern margin of the Tibetan Plateau.