The collapse of unstable rock is actually the problem of fracture and extension in dominant fissure of unstable rocks. Based on fracture mechanics model of dominant fissure, this paper puts forward a fracture stability determination method for unstable rocks, and establishing stability coefficient by the index of union stress intensity factor. By this method tip stress distribution forms of dominant fissure and its calculation expression are deduced. Fracture mechanism model of tip in dominant fissure for unstable rocks is proposed; and then for this calculation method union stress intensity factor in tips of dominant fissure is deduced. Fracture stability calculation results of unstable rocks in Shouli Mountain, Wanzhou Area, Chongqing show that the calculation results fit well with the field situation, so as to show that the fracture stability calculation method proposed is more security, objective and practical than the existing “specification method”.