Abstract: Claystone is a common geotechnical medium in deep underground engineering project. In order to ensure the safety of tunnel during its excavation,the work which is related to its mechanical properties is required to be studied. The existing test results show that the claystone exhibits some obvious strain-hardening,strain-softening and plastic flow characteristics during the triaxial compression test. At the same time,it is found that the samples produce some obvious defects during the loading process,and the existence of these defects would weaken the mechanical properties. In order to establish a model which can describe the mechanical phenomena of claystone,the micro-crack is used as the damage element to construct a reasonable plastic damage variable. The damage variable is introduced into the modified Drucker-Prager cap model together with the plastic hardening variable to construct a new constitutive model. The model is numerically implemented by the ABAQUS platform and its UMAT subroutine. In the subroutine,the plastic hardening and damage variables are introduced into the iterative calculation based on the backward Euler constitutive integral algorithm,so that the yield surface is updated with the stress in the iterative process until the stress returns to the yield surface (The size of the yield surface changes with the plastic damage and hardening variable). This model is used to simulate the triaxial compression test of claystone under normal temperature,and the simulation results are compared with the experimental results. The comparison results show that the simulation results are in good agreement with the experimental results,indicating that the constitutive model established in this paper can better reflect the me- chanical properties of claystone. Finally,the plane excavation model of claystone tunnel is established. The new model is used to simulate the excavation process. The evolution of damage variables,stress and pore water pressure during the whole simulation process is analyzed. The simulation results verify the applicability of the new model in numerical sim- ulation of underground tunnel excavation.