Abstract: The Zhaojiayan rock fall is a typical rock fall which occurred at the mountainous area in western Hubei. Field investigation shows that steep terrain, fissured rock masses and gentle weak intercalated layer are the basis of dangerous rock masses formation; karstification and coal mining activities are the influence factors which had resulted in deformation development of dangerous rock masses; and heavy rain is the direct incentive of rock fall occurrence. In order to find out the formation mechanism and failure mode, critical water head height in the back tensile crack are calculated with the rigid block limit equilibrium method for potential failure modes on the basis of qualitative analysis of formation conditions and influence factors of the rock fall. The collapse failure mode is discussed by comparison analysis. The results show that the underlying carbonaceous shale has been sheared as the dangerous rock masses are pushed out by the water pressure which is filled in the back tensile crack, and the sliding rock fall happens. The analysis results can be used as a reference for analysis of similar rock falls and prevention occurred in the mountainous areas in western Hubei.