Based on the results of compression test of marbles from Jinping Ⅱ hydropower station, an expression of plastic internal variable considering the volumetric stress effect is derived, and the evolutions of strength parameters and dilatancy with the internal variable are studied. The results show that: the cohesion of marbles decreases slightly in pre-peak stage while the cohesion decreases rapidly in post-peak stage; the internal friction angle increases with increasing internal variable and it reaches the maximum value when the internal variable reaches 0.8-0.9; the dilatancy angle increases firstly and then decreases with increasing the internal variable under different confining pressures, and the variations of dilatancy angle are different under low and high confining pressures. A mechanical model of marbles is established considering the effect of volumetric stress based on the above results, and the laboratory triaxial compression test is simulated with the established model using FLAC3D. The simulation results show a good agreement with the test results, indicating that the established model can reflect the main mechanical properties of marbles. The research methods and results will provide an important reference to the analysis of deformation and failure of other deep brittle rock masses.