Abstract: In order to understand the failure mechanism and mechanical properties of soil-rock mixture, the shear characteristics of soil-rock mixture were studied by PFC numerical simulation test, mainly including the influence of particle size mass fraction of different particle gradations, particle loss and model size on shear strength. The results showed that the amount of fine particles played a vital role to shear strength in the model test. The greater the fine particle content, the greater the shear strength of the model, otherwise the opposite. The particle size mass fraction of fine particles of 85% was an obvious boundary point to distinguish the jump of the shear stress-shear displacement curve. There was a negative correlation between the model shear strength and the model area, and the smaller the mass fraction of fine particles, the stronger the correlation between the model size effect and the shear strength.