Abstract：The previous on hydraulic fracturing generally assume that tensile cracking is the essential mode of hydraulic fracturing in rocks. In reality，however，the mode of fracturing is combined with compression and shearing，as observed in many rock engineerings. In this paper，the effect of hydraulic fracturing on deep-lying hydraulic tunnels under high water pressure is studied on the principles of fracture mechanics. Firstly，the analysis method of stress intensity factor of crack of model I in many complex conditions in the surrounding rock masses of tunnels is presented. Then，the criteria of hydraulic fracturing of different tensile cracks are discussed. The mechanism and formula for stress intensity factor are put forward. Finally，by using the above theory，the hydraulic fracturing analysis of under-lying hydraulic tunnels of a hydropower project is performed，which shows that the method presented is reliable.
