Abstract: The hydraulic fracture technology is employed to estimate the in-situ stresses and other relevant mechanical parameters of rock mass at a new-type diversion hydroelectric project in China. The measured results show that the engineering area is under the stresses with the orientation of NNW, and the maximum principal stress is from 7.35 to 8.16 MPa in magnitude, with the direction of N33°～52°W and the inclination of 26° to 29°. The maximum principal stress around the air-cushion pressure adjustment cavern is 10.63 MPa, and the minimum principal stress is 4.98 MPa; the high values of anti-cracking strength of wall rock mass are above 5.50 MPa, and the low values are between 3.00 and 3.50 MPa. According to the general law of stress nonhomogeneity in the wall rockmass, and considering the respective characteristics of measured in-situ stresses and anti-cracking strengths of fissures in wall rockmass, a new method is put forward to estimate the minimum principal stress in the wall rockmass, which incorporates the in-situ stress measurements, in-situ bearing capacity measurements, and elastic modulus data. By use of the new method, the estimated minimum principal stress is 2.86 MPa for the area around the air-cushion pressure adjustment cavern. Finally, the impacts of in-situ stresses on the underground factory caverns, water tunnels and air-cushion pressure adjustment cavern are discussed.