Abstract: Based on the spatial network model developed by our team， the general theory of block was used for block analysis， the safety factor， the volume and the edge length of unstable blocks in the excavation surface were obtained. The block generation probability can be first determined by the edge length and the trace length， then the block failure probability was calculated by the block generation probability and the safety factor. The failure block was identified when the failure probability of block is greater than the standard value. The ratio of the failure volume of rock blocks to the unstable volume of rock blocks was used as an evaluation index on the excavation surface stability， and a stability analysis method of the excavation surface was proposed on the basis of the unstable blocks failure probability. An example was given to evaluate the stability of the excavation surface based on this method.

Key words: unstable block, failure probability, excavation surface, stability, structural plane