<p>Many natural geological disasters on earth, such as landslides, debris flows, and rock avalanches, are closely associated with the large flow deformation of granular particles, posing significant threats to life and property. During such granular flow deformation, a substantial number of particles undergo breakage, leading to significant acceleration of mass motion behavior. In this study, a ring shear apparatus and acoustic emission instrumentation were used to investigate the evolution characteristics of particle breakage and its impact on the apparent viscosity of quartz sands under large shear deformation. The results indicate that the apparent viscosity decreases significantly with increasing shear distance, and such decrease is more pronounced under higher normal stress, primarily due to the increase of the relative breakage of the particles. The impact of shear rate is predominantly observed in the apparent viscosity, but minimal influence was found on the relative breakage index and particle shape. The intensity of AE energy peak consistently precedes the shear peak intensity, making AE energy signals as a valuable tool for monitoring particle breakage. Particle shape analysis demonstrates the importance of examining the shape of fragmented particles to explain the changes in the apparent viscosity and the underlying mechanism of volumetric shear-dilatancy and contraction. The findings highlight the significance of evaluating the effect of particle breakage in analyzing the mechanism of high-speed long runout landslides. These insights contribute to a better understanding of granular material behavior and its implications for natural hazard assessment and mitigation.</p>