Latent crack has no obvious effect on fracture process in the shallow rock engineering. As the project entered into deep part, there appeared many phenomenon which is different from that in the shallow rock engineering, and latent crack plays a more and more important role. By analysis of micro defects of rock and numerical method, potential paths of crack propagation are prefabricated, and adjust the quantity and spreading direction of latent cracks, then by using the numerical test result and mathematical statistics, the paper analyzes the function of latent crack of different level in the process of fracture. The results show that fracture strength and latent crack of rock have close relationship with each other, and rock fracture strength is related to the spreading direction of latent crack, and the smaller the angle between latent crack and major fracture crack is, the greater effect it is on rock fracture strength. As the number of latent crack increase, the effect of the spreading direction of latent crack on rock fracture strength decreases, and the influence degree is depends on their connectivity degree among latent cracks.