The loading/unloading response ratio theory is introduced to analyze the seismic response of underground rock cavern group. The response region is determined reasonably by taking the input seismic accelerations as loads and unloads and cavern accelerations as response. The numerical section model of No.13 unit of Baihetan hydropower station is established by FLAC3D. With Wenchuan earthquake as input wave

the seismic stability of underground rock cavern group is discussed. The result shows that the peak value of loading/unloading response ratio is about 4.05-11.52 under 2% exceeding probability in future 100 years. The rock mass around top arch of main powerhouse and interlayer C4 in upstream wall of tailrace surge chamber are under the state of nonlinear deformation

but the loading/unloading response ratio is not infinite during the whole process

with no stability failure of rock mass. The relative displacement of interlayer has adverse effect on the wall stability of tailrace surge chamber. This research method can be used for seismic stability study of common rock mass.