According to the theory of fracture mechanics, the mechanical model of rock-mass with multi-cracks under the action of seepage pressure was set up. Based on the principle of superposition, the formula for calculating stress intensity factor (SIF) was built. The stress intensity factor of cracks in different conditions, such as different crack distance, different crack angle and different seepage pressure, were discussed. It is suggested that, if distance between two cracks is short, the interaction of cracks has significant influence on stress field of crack tip. According to the change of relative crack location and crack angle, the different superposed stress can produce two effects, stress field strengthen and stress field weaken. The stress intensity factor would be increased when the crack tip is in the stress strengthen area; otherwise this value would be decreased. The interaction of adjacent cracks is related to the distance between two cracks. As the crack distance increases, the SIF is close to a fixed value which is calculated without considering the interaction between cracks. Therefore, when the crack spacing is larger than crack length, the interaction between adjacent cracks can be ignored. The results also show that the seepage pressure significantly affects the stress field near crack tip, and the stress intensity factor KI increases along with the increase of seepage pressure (-KI decreases).