Abstract: The Jiweishan event happened at June 5, 2009 in Wulong county of Chongqing municipality of China. It is a mega sliding rockfall(volume is larger than 100104m3),and an oversize geological disaster(the number of missing and death is greater than 30 in Chinese rule). It is a basic condition in substance and structure that Jiweishan rockmass is of layered limestone,weak interlayers and fissures approximately in SN and EW direction. Raining seepage and karstification in a long period may decrease strength of weak layers in the bottom of the rockmass and enlarge fissures in the mountaintop,so that layered rockmass is pulled out easily. Based analogical analysis,Jiweishan dangerous rockmass may be result from pull cracks in the mountaintop produced by cantilever plate effect which derives from great iron ore mined out area under mountain. It is obtained from the sliding force analysis in apparent direction that outward apparent sliding force makes rockmass in crawling into rockfall gradually by through to overcome a friction force from its bottom and a resistant shearing force on its front end. The Jiweishan rockfall disaster is induced by iron ore mined out area and the apparent direction sliding force together. It is a process in chain reaction that consists of rockmass cracks～creep sliding along weak layer～bursting rockfall～debris impact～come into being disaster. The dynamical source of the event is explained for emergency management decision and a simple method in technical support for similar problems is put forward in this paper.