Rock avalanches frequently lead to catastrophic consequences due to their unpredictably high mobility. Numerous researchers have studied the shear behavior of granular materials under various conditions, attributing the high mobility to ultralow resistance. However, the underlying physical mechanism of frictional weakening remains unclear. This study utilizes the discrete element method (DEM) incorpo-rating the fragment replacement model to simulate plane shear flows under various normal stresses (0.2-1.2 MPa) and shear velocities (0.01-2 m/s). The findings reveal a localized shear band characterized by a J-shaped velocity profile and high granular temperature, and a concentrated distribution of weak contact forces forms at a shear velocity exceeding 0.1 m/s and normal stress above 0.6 MPa. Moreover, frictional weakening is observed with increasing normal stress from 0.2 MPa to 1.2 MPa and increasing shear velocity from 0.1 m/s to 2 m/s. The evolution of the steady-state friction coefficient can be divided into two stages: an initial stage (I) and a weakening stage (II). During stage I, the steady-state friction coefficient slightly increases until reaching a peak value. However, upon entering stage II, it gradually decreases and approaches an ultimate value. The velocity-and normal stress-dependent frictional weakening can be attributed to shear localization and embedded packing structure induced by particle breakage, respectively. Finally, an optimized m(I) model is proposed to capture the full evolution of the friction coefficient with the shear strain rate, which can improve our understanding of rock avalanche dynamics. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).