Taking the defective rock as the research object, the number and angle of cracks are generated by discrete element software, and the number and distribution mode of cracks and the influence of confining pressure on the mechanical properties and failure mode of rock are studied. The results show that with the increase of the number of rock cracks, the compressive strength of shale shows a negative exponential change, and the elastic modulus shows a linear decrease. With the increase of the number of cracks and confining pressure, the number of new cracks corresponding to shale failure decreases. The new crack angle is mainly concentrated at about 90 °, which obeys the normal distribution. The confining pressure has an inhibitory effect on the asymmetry of new cracks. The primary cracks perpendicular to the direction of the maximum principal stress have a shielding effect on the stress, but the confining pressure can effectively reduce this stress shielding effect. A mechanical evaluation method of damage variable is established to quantitatively analyze the relationship between the number of cracks and rock damage variable.