Abstract: For an underground structure, long-term shear strength of the surrounding rock always concerns engineering designers and researchers. In fact, the shear strength tested within a limited time period does not truly reflect its long-term strength. Therefore, it is very important to conversely calculate the long-term shear strength of rock pillar (including the base of pillar) of large ancient underground caverns, based on its shearing failure. In this paper the back analysis of long-term shear strength of no.4-2 rock pillar in the no.4 cavern at Longyou Caverns is presented. In order to further consider distribution of stresses in different parts of the shearing surface, the authors used FLAC-3D in simulation. The results shows.