Abstract: In order to study the failure rule of cracked rock under the unloading condition, on the basis of triaxial compression test using ideal rock with precast single crack whose angle is π/4, we use Mohr - Coulomb criterion to analyze the failure process of cracked rock, and obtain the broken rule of single cracked rock by calculation in the ideal condition. According to the calculation, the broken process is divided into three stages. In the beginning of unloading, rock deformation is the elastic deformation, so the rock strength is little affected by the cracked; in the middle of unloading, the crack is further broken and crack damage is given priority to expansion, and the stress concentration is emerged at crack tip; in the end, the new crack is produced in rock and the precast single crack is further broken until the rock can not afford the stress.