On the basis of the fracture failure mechanism and stress distribution characteristics of compressive-shear rock cracks under biaxial compression, this paper theoretically examines the influence of seepage pressure on the effective normal stress of preexisting crack surface. Then, a judging criterion is proposed for determining whether the seepage pressure is higher or lower. It considers the stress state of the effective normal stress of preexisting crack surface. Finally, the cracking law and characteristic of microcracks, generated at the tips of preexisting crack with different seepage pressures, are studied using the maximum circumferential stress criterion and sliding crack model. The results show that the optimal inclination angle of preexisting crack (on which the initial cracking strength of fissure body is the minimum) is directly related to the frictional coefficient of preexisting crack surface under biaxial compression. The optimal inclination angle of preexisting crack increases from 45° with the increasing of frictional coefficient of preexisting crack surface. When the seepage pressure is lower, the frictional coefficient of preexisting crack surface is weakened because of presence of seepage field. Furthermore, the optimal inclination angle of preexisting crack is close to 45°. On the other hand, the seepage pressure directly is applied to the surface of preexisting crack. Therefore, it just influences the initial cracking strength of fissure body. When the seepage pressure is higher, the stress state changes from compression-shear to tension-shear on the surface of preexisting crack. Because of the action of tensile stress, the initial cracking angle of microcrack generating at the tip of preexisting crack increases from 70.5° to 0° with the increasing of the value of KI /KII.