Abstract: According to the fundamental fact of the constitution of falling ground in laneway, the inter-set relationship between the intersection of the structural traces and the excavation traces and dangerous rock is discussed, the possibility space of falling height in laneway is established ,the condition probability of every intersection relationship of the structural planes and the excavation planes is analyzed, and the possibility formula is induced combining with the distributed function.In practical application, not only the distributed possibility is solved, but also the sensitive factor of falling height is analyzed.The scientific basis of support design of the laneway, especial the length of supporting bolt-length in laneway roof is provided.The decision-making gist of macroscopical safety management,choosing the minimum falling height and the laneway trend of the minimum falling possibility is provided.