The rock mass of open mine slope is the 3D damage mass, which contains a large amount of joints distributing randomly in space. Because both its strength and joints distribution are random, the damage of rock mass is a kind of probability damage. According to the definition of damage tensor of jointed rock mass and simulation of 3D joint network, the analysis approach and program of the damage tensor and the probability distribution rule of jointed rock mass are put forward. Based on the Rosenblueth principle, the theories of effective stress in damage mechanics and fracture mechanics, FLAC numerical simulation and 3D joint network simulation, etc., the coupling analysis technique of the probability damage evolution in 3D jointed rock mass has been established. It organically combines the damage theory, fracture analysis of jointed rock mass and numerical simulation of slope excavation, and provides a practical approach for the probability damage evolution rule of jointed rock mass. The formation of slope is a long-term process during mining. With the development of mining engineering, damage of jointed rock mass is progressively accumulated. The results have been proved during the excavation of the south slope of Yanqianshan Open Iron Mine. The assessment and working process of the probability damage and the evolution are systematically studied with the excavation of the rock slope. The spatial distribution and evolution of the damage rules of the rock slope are quantitatively shown. 3D random damage fields of the rock mass in different excavation stages are established by applying Rosenblueth principle and numerical simulation of the damage evolution. The damage tensor, the mean and the variation of damage degree are calculated. The dynamic laws of stability and reliability of open pit mine will be well studied.