Abstract: In August 2010,serious deformation occurred in the Wangxia dangerous rock mass.The local residents and river channel safety are in high risk.This paper is based on detailed investigation of the new cracks,deformation history,GPS observations,total station monitoring and crack displacement monitoring data analysis.It analyses comprehensively the deformation and reasons of the unstable rock mass.The results show that the overall displacement direction of the rock mass was towards to SW with obvious subsidence deformation.The T10 crack controlled cracking and the back edge of the deformation zone that mainly manifested as crack opening and settlement deformation.The overall displacement direction of the block between the T11and T12 cracks was towards to SW and accompanied with settlement deformation more than horizontal displacement.Serious deformation occurred in the block near the T13 crack,caused by the block squeezing action between the T11and T12 cracks.A group of structural planes with large and straight persistence and good apertures were developed due to connected joint surfaces in the formations of the dangerous rock mass.The planes controlled the destruction of the rock that were parallel to the failure surface.The dangerous rock mass and its weak base rock formed a typical hard to soft to hard lithologic layers,which provided a combined condition of the geological structure for the interlayer-gliding of the rock mass.The expansion of the cracks and collapse pits were formed on the basis of previous deformation.They had very close relationship with the mining activities at the bottom of the rock mass.Subsided extrusion type collapse had formed so that the dangerous rock mass undertook rotational sliding to the free face after rainfall.