Abstract: The Baihetan hydropower plant is one of the most important backbone energy points of power transmission from west to east in China’s energy layout. In the initial stage of hydropower plant construction, some in-situ stress data were obtained by using conventional methods. However, with the continuous excavation of underground cavern, significant brittle failures caused by high stresses were observed in the surrounding rock. Therefore, the study on the stress field of surrounding rock mass is highly important for the long-term stability control of underground caverns. The borehole wall was scanned by the high-precision ultrasonic downhole television imaging system to obtain the logging image, and the widths of breakouts WBO was extracted by WellCAD software. The stress polygon based on Mohr-Coulomb criterion and Anderson theory and the breakout failure method based on Wiebols-Cook criterion were integrated and utilized to limit the estimation of stresses value, the characteristics of rock mass stress field in the project area were analyzed. The results show that the orientation of the maximum horizontal principal stresses in the engineering area was about N30.15°E, and the magnitudes of the major principal stresses were Sh=11.62±0.34 MPa (horizontal minimum principal stress), SV=14.45±0.33 MPa (vertical principal stress), SH=25.88±5.61 MPa (horizontal maximum principal stress) respectively. This stress state is favorable for the strike-slip faulting(Sh

Key words: Baihetan hydroelectric power plant, ultrasonic borehole televiewer, stress polygon, borehole breakout method, stress field of engineering rock mass