Abstract: A three-dimensional Voronoi polycrystalline discrete element model comprising an assemblage of Voronoi polycrystal generated by open source software package Neper was built using a distinct element code(3DEC)， which considered mineral grain composition. The mechanical input parameters of sandstone at a mesoscale were calibrated based on the conventional triaxial compressive testing data in laboratory， and the mesoscopic damage evolution of sandstone under triaxial compression was investigated. The results showed that the microcracks under low confining pressure are mainly tensile cracks， accompanied by a small number of shear cracks， while the number of shear cracks under high confining pressure increases significantly， and even is larger than that of tensile cracks before peak strength. The evolution of the microcrack in the sample reflects the transition of the sandstone from brittle to ductile under confining pressure. The sandstone appears obvious dilatancy phenomenon. The larger the confining pressure， the more obvious the dilatancy lags. With the increase of confining pressure， the ratio of the stress corresponding to the dilatancy point to the peak strength increases.

Key words: Voronoi polycrystal, 3DEC, heterogeneity, triaxial compression, mesoscopic scale