Abstract: The existence of fractures has a considerable influence on the mechanical properties of hydropower high and steep rocky slopes. The method to construct an equivalent rock mass calculation model that reflects the distribution characteristics of three-dimensional (3D) fractures is the key to analyze and evaluate the mechanical properties of rock mass. Based on the theory of damage mechanics and statistical strength, this study proposed a new method to calculate the equivalent rock 3D random fractured network model using the 3D rock failure process analysis (RFPA^3D). First, based on the Baecher model and Monte-Carlo method, the reconstruction of the 3D random discrete fracture network (DFN) model was implemented in the RFPA^3D. Furthermore, an equivalent rock 3D random fractured network model of engineering scale was constructed using the embedded DFN model and through giving different mechanical parameters to fractures and rocks. All the types of load combinations can be applied to the model to realize the analysis of the mechanical properties, such as failure process, deformation, and the strength of the 3D random fractured rock mass. The rock mass downstream of the dam site area of the left bank slope of the Lianghekou Hydropower Station was then taken as the background, and the geometric parameters of the joints in the region were analyzed and studied using the 3GSM software. Moreover, the characteristic values and the distribution types of joint geometric parameters in the study area were obtained. Finally, the accuracy of the 3D random DFN model was verified by taking the fractured rock mass downstream of the left bank slope dam site of the Lianghekou Hydropower Station as an example, and the size effect of the fractured rock mass in the study area was studied. Results show that the representative elementary volume of the jointed rock mass is evaluated as 8 m × 8 m × 8 m, and the corresponding equivalent uniaxial compressive strength and elastic modulus are 25.159 MPa and 19.443 GPA, respectively. Research results provide a new method for the study of the mechanical behavior of the equivalent rock mass.