Abstract: Rock bed scour downstream of high dams is mainly due to propagation of the fluctuating pressure within rock mass fractures. The fractures network is the storage of groundwater， and its structure determines the propagation law of the fluctuating pressure. In this paper， applying the Monte-Carlo simulation method， a simulated network of rock fractures is obtained by compiling a Fortran program. The simulated network is equivalent to the real rock fractures network in the aspect of statistics and has similar structure characteristics with the real network. Meanwhile， the cells′ and nodes′ numbers are automatically added to the network. Based on the matrix theory， the network is accurately represented by the joint matrix and the loop matrix. In order to reproduce the bed scour process， new methods of isolated block auto-searching and rock layer auto-tracing are proposed in the study. In this paper， as an example， two groups of the rock mass fractures in the area of 10 m×10 m are simulated by the Monte-Carlo simulation method. The figure of the fracture network is given and the results of rock layer auto-tracing at different scour stages are given by the Monte-Carlo method. These analysis results have laid a solid foundation for numerical simulation of riverbed scour process at the downstream of high dams.