Abstract: Recently, the research of basin-mountain system has become one of the hotspots of the frontier research fields in the study of continental geodynamics. For deeper understanding tectonic evolution of Dabashan foreland and to resolve key problem of petroleum exploration this area, a deep seismic reflection profile with a length of 300km was conducted in 2007 by cooperation of Chinese Academy of Geological Sciences (CAGS) and Southern Exploration and Development Division Company of SINOPEC (SEDDC). Based on the geometry of lithosphere structure from deep seismic reflection profile and the velocity data from Deep Seismic Sounding profiles (DSS), the authors have made an analysis on the feature of lithosphere structure of northeastern Sichuan-Dabashan basin-range system and some discusses on the relationship between deformation style and lithosphere structures, and then update understanding Dabashan orogen. The basement of Yangtze craton extends beneath whole northeastern Sichuan-Dabashan basin-range system. The larger thickness and strength of lithosphere result in Yangtze craton focus its contract deformation rather at the top of crust than middle-to-lower crust in the process of intracontinental orogeny after Yangtze spliced with North China craton. Due to regional deep decollement decoupled the sedimentary cover and basement, the crystalline rock crust keep rigidity and just behave in large scale bending hardly evident lateral shorting,so that the Dabashan orogen shows us feature of 'thin-skinned structure’ and rootless. The deformation styles and involved depth of Dabashan orogen is dominated by a regional deep decollement existing within the base of mudstone rock layer of lower Cambrian system and marked by TWT 4.0 second reflectors. Along this decollement, the Chengkou fault thrust the strata Sinian System-Lower Paleozoic to shallow and surface moreover overthrust them about 60km to SW onto strata Mesozoic-Paleozoic of northeastern Sichuan basin. The Zhenba fault and Chengkou fault also get gently and vanished at it in deep. The underlying metamorphic rock layers are nearly not involved into deformation.