Abstract：The creep properties of granite specimen are studied under triaxial compression creep experiment with the method of step loading，and the experimental results show that there exists a stress threshold in the creep deformation of rock from stable creep phase to accelerating creep phase. If the applied load is lower than the threshold，the initial microcracks and pores in rock are compacted under the lower stress level and the microcosmic linear visco-elastic deformation will come into being in the microstructure of rock，so the creep rate of rock is uniform and the deformation will tend towards a steady value，and the rheological parameters are invariable under the lower stress level；when the applied load is bigger enough to exceed the threshold，a mass of microcracks come into being，grow quickly，and the damage evolvement is rapid，this will induce the change of rheological parameters of rock. To obtain the value of this stress threshold，the triaxial compression creep experiment of rock is simulated with the method of three-dimensional numerical simulation and the stress threshold is obtained. The damage variables are introduced into the generalized Kelvin model，and the corresponding damage evolution equations which are able to reflect the characteristics of the accelerating creep phase of rock are set up. Furthermore，by applying the bifurcation theory of solid mechanics，the bifurcation behavior of rock due to the change of rheological parameters after the deformation turning into the accelerating creep phase is studied，the rheological parameters，which cause the creep bifurcation behavior of rock，are determined and the bifurcation points are obtained. The results show that the calculated curve based on the effective initial elastic modulus considering damage factor agrees well with the experiment curve.
