Deep diversion tunnel of Jinping Ⅱ Hydropower Station is located in a high stress zone; for its surrounding rock

i.e. marble

the conventional triaxial test and pre-peak unloading test are carried out. Based on pre-peak unloading test of the marble

its deformation

parameters and strength characteristics under pre-peak unloading are studied; and a power function Mohr strength criterion which can fit the pre-peak unloading tests appropriately

is established. The conditions of pre-peak unloading in numerical simulation is given. By using finite difference procedure of FLAC3D

a numerical simulation model is established; and it is analyzed and compared with the laboratory test result. Finally

the characteristics of the pre-peak unloading rock mass is acquired. The results indicate that nonlinear Mohr strength criterionis can fit to the strength characters of the rock under pre-peak unloading test; at the beginning of pre-peak unloading test

elastoplasticity characteristics is shown; the peak strength and decline of the confining pressure increases continuously as the confining pressure increase. These conclusions reveal the unloading mechanical properties of marble under high geostress and provide reliable theoretical reference for us.