The existence of fractures has a significant influence on the mechanical properties of hydropower high and steep rock slopes. How to construct an equivalent rock mass calculation model that reflects the distribution characteristics of three-dimensional fractures is the key to analyze and evaluate the mechanical properties of rock masses. Based on the theory of damage mechanics and statistical strength, a new method to calculate the equivalent rock three-dimensional (3D) random fractured network model is proposed on the basis of 3D Rock Failure Process Analysis (RFPA3D). First, based on the Baecher model and Monte-Carlo method, the reconstruction of the three-dimensional random fractured network model (DFN) model is implemented in the RFPA3D. Then, using the embedded DFN model and giving different mechanical parameters to fractures and rocks, an equivalent rock 3D random fractured network model of engineering scale is constructed. the analysis of mechanical properties such as failure process, deformation and strength of 3D random fractured rock mass was realized. Finally, the accuracy of the three-dimensional random DFN model is verified by taking the fractured rock mass downstream of the left bank slope dam site of Lianghekou Hydropower Station as an example, and the size effect of fractured rock mass in the study area is studied. The research results provide a new method for the study of mechanical behavior of equivalent rock mass.