Abstract：According to the fundamental condition to form a falling block in underground engineering，the interset relationship of structural plane traces and excavation traces is analysed. Then a possibility space of falling blocks is found. Using the random distribution character of structural plane in rockmass，the forecasting formula of falling space possibility is obtained. By analyzing engineering cases，the possibility of falling height and falling volume，and the curved surface of falling space possibility are given，which changes with the variation of the interval distance of structural planes.
