Abstract: Through the thin section identification and micro-characters study.Cracked rock mass are mainlyillite clay with shaly structure,thin-layer structure and joints were developed.Through the analysis of regional tectonic features,the regional tectonic process controls the formation of cracked rock mass.Yan Hecheng fault formed fracture network of the region and cracked rock mass after repeated loading-unloading process by extrusion,tension and squeeze and Zi Jing Guan fault,Fang Shan-Ba Bao Shang fault twist action.Rock mass normal unloading and gently dipping structural plane unloading rebound so that rock mass become more cracked.