In order to study the law of crack initiation, propagation and failure in rock,, using PFC2D numerical simulation software with indoor test analysis of precast single fracture rock sample. The results show that: (1) When the confining pressure is 7MPa, the shear-tensile composite failure mode is the main failure mode; When the confining pressure is 14MPa, the specimen mainly adopts "Y" shear failure mode and single oblique shear failure mode; When the confining pressure is 21MPa, the specimen mainly adopts the "X" shear failure mode. (2) The failure mode of the sample is closely related to the confining pressure. When the length of the crack is constant, the failure of the sample becomes more obvious with the increase of the confining pressure. When the confining pressure is constant, the scale of crack propagation increases with the increase of crack length. (3) The peak strength, elastic modulus, crack initiation stress and damage stress of rock samples all decrease with the increase of crack length; With the increase of confining pressure, all mechanical parameters increase, and with the increase of crack length, it is more likely to induce new cracks, and the strength characteristics of samples decrease. (4) Shear cracks play a dominant role in the whole simulated loading process and their number is always large; With the increase of confining pressure, the tensile crack gradually plays a dominant role. When the confining pressure is 21MPa and the crack length is 39mm, the tensile crack is 2.73 times of the shear crack. When the confining pressure is constant, the proportion of tensile cracks decreases gradually with the increase of crack length.