Abstract: There are many mountains in Guizhou Province. Due to the characteristics of karst landforms,collapses,landslides and other geological disasters occur frequently,which is one of the most serious geological disasters in China. There were many collapses in the development process of Xujiadabang collapse in Sinan County. The existing dangerous rock mass is on a large scale and a serious threat to the life and property safety of Xujiadabang residents. So it is urgent and necessary to carry out corresponding risk assessment. In this paper,the high precision DEM of Xujiadabang collapse area after vegetation removal was obtained with UAV Lidar technology. The 3D terrain model was constructed by 3D real scene modeling technology. Considering the situation of rock fall from two empty surfaces,three typical particle sizes of 0.3 m,1.5 m and 2.5 m were selected for random numerical simulation. The motion trajectories and dynamic characteristics of three kinds of particle size were obtained. Four evaluation factors of falling rock energy,frequency,acceleration and bouncing height are selected to comprehensively evaluate the risk of collapse in Xujiadabang. The weight of each evaluation factor is calculated with entropy weight method. The risk range distribution of Xujiadabang collapse is obtained using spatial grid analysis technology based on ArcGIS software. The collapse risk range of Xujiadabang was divided into four grades: extremely high risk area,high risk area,medium risk area and low risk area. The research ideas and technical methods adopted in this paper can provide reference for the risk assessment and scope division of other monomer collapse disasters.