Abstract:On the basis of the characteristics of surrounding broken rock mass of underground constructions,by using the program of‘discontinuous deformation analysis(DDA)’,the changing law of influential factors on displacement of discontinuous broken rock mass is studied quantitatively for the first time. According to the results of computing,the concept of‘key part’of underground constructions and its stability criterion are brought forward. It is pointed out and it has been advanced that in tilted coal and rock seams,the‘key parts’of underground constructions are the floor and especially the upper side of surrounding rocks.