This paper studies the influence of different boundaries on the conventional triaxial simulation test of granite. The particle flow code（PFC3D） was used to generate the granite model based on the method of grain?-based model. At the same time， the discrete element triaxial test models were established respectively for the three boundaries of the rigid membrane， flexible membrane and finite element （“Shell”） by using the FDM?-DEM coupling modeling technology. Test results shows that compared with the “Shell” boundary， the rigid boundary will increase the probability of stress concentration in the sample and affect the failure form of rock samples， the flexible membrane boundary will cause the actual confining pressure of the rock sample to be larger than the target confining pressure during the triaxial loading process， and significantly increase the residual strength of the rock sample. However， the confining pressure of the rock sample under the shell boundary is stable， and the probability of stress concentration is the smallest. Therefore， the shell boundary should be used in the triaxial test of discrete element simulation.