Engineering rock mass is commonly a brittle medium containing lots of joints or fissures. Under the stress redistribution in construction，the crack initiation，propagation，and coalescence may cause the strength and stiffness degradation of such medium. And these have an important impact on the stability of rock mass. By employing the analysis software of FLAC3D and improving the constitutive relation, the failure process of the double-cracked rock specimen under uniaxial and two-dimensional compression are simulated and studied. The numerical results match well with the testing results obtained by former scholars. The strength envelope of the jointed rock is obtained as well, and it is applied to analyzing the stability of a slope project.