Two co-seismic surface rupture zones, 230 km and 70 km long, are produced by the 2008 Wenchuan Ms 8.0 earthquake along the pre-existing Yingxiu-Beichuan fault and the Guanxian-Anxian fault, respectively, in the Longmen Shan fault zone. The post-earthquake geological study shows that the exposed fault planes accompanying with co-seismic scarps usually occurred along the coal seams and carbonaceous shales. In this paper we present the observed results of fault scarps and the slip surface in some typical locations along the co-seismic rupture zones. Comparing with the 1:50000 regional geological map, it shows that the distribution of the major section (the southwest part) of the Yingxiu-Beichuan co-seismic rupture zone and the Guanxian-Anxian co-seismic rupture zone are corresponding with the exposed area of the Upper Triassic Xujiahe coal-bearing strata in the Longmen Shan region. The Upper Triassic Xujiahe coal seam and the Middle-Lower Triassic and Silurian, Cambrian shales in the Longmen Shan region are the flexible rock layers, which are easy to slide and form detachment surface or detritus within the fault zone. The co-seismic rupture zone produced by the May 12 Wenchuan Ms 8.0 great earthquake resulted from a B-type sliding along the flexible rock layers in the shallow crust level.