Abstract: Granular materials are widely used in civil engineering and it is crucial to clarify the deformation evolution of granular material systems. The Discrete Element Method (DEM) is used to simulate the direct shear test of granular materials. The shear strain is assumed to be a combination of the particles’ rotation and slip. The deformation evolution of the granular material system during shearing process is investigated with the rate-state shear strength theory for granular material. To study the effect of particle rotation on the shear strength of granular material system, a 3D discrete element simulation of a constant-shear-rate direct shear test using spherical glass bead system is carried out by PFC^3D. The ratio of the slip caused by the particles rotation to the total slip in the system is obtained. The results show that the particle rotation has a significant contribution to the internal structure evolution of the granular material system, and the slip caused by the particle rotation cannot be ignored. The evolution of particle rotation is obtained by monitoring the particle rotation angle around Y-axis in the shear band. For the vertical stresses applied in this study, the slip caused by particle rotation decreases as the vertical stress increases. With the increase of shear displacement, the slip caused by particle rotation decreases gradually and tends to be a constant. Based on the numerical analysis on rotation evolution of the granular material system, the rate-state shear strength model of the granular material system is further verified and the corresponding method for determining the key model parameters is also given, providing a basis for analyzing and determining the strength of the granular material system.