Abstract: The Mohr-Coulomb and Hoek-Brown failure criteria are the two most widely used ones at present. To simulate brittle failure of rocks in deep tunnels, the models based on these two failure criteria have been proposed, including the CWFS (cohesion weakening and friction strengthening) model, the DISL (damage initiation and spalling limit) model, and the BDP (brittle ductile plastic) model. These models have been used to simulate brittle failure of hard rocks. However, because of the issues of strain hardening under high confinement and large ambiguity in model parameter determination, it is very challenging to apply these models in practical engineering application. Based on the variation of the GSI (geological strength index) value during compression of rocks and by defining the GSI value as a function of plastic strain and confinement, a strain-dependent GSI-softening model, which is based on the Hoek-Brown failure criterion, is proposed in this study. This model is implemented in FLAC^3D to simulate the triaxial compression tests on T_2b marble in Jinping-II Hydropower Station and the Tennessee marble. It is found that the proposed model is able to simulate the mechanical behaviors of the brittle-ductile transition observed in the triaxial compression tests on Jinping and Tennessee marbles. Finally, this model is used to evaluate the relaxation depth of a headrace tunnel in Jinping-II Hydropower Station, and the calculated depths are in good agreement with the field observations.