Abstract: The shear strength of rock-soil mixture materials is affected by the content, spatial distribution and shape of rocks. In this study, soil particles are simulated by spheres, and irregularly-shaped rock particles are constructed by overlapped spherical particles. Direct shear tests are simulated with the discrete element method under various rock contents and spatial distributions of rocks. The shear strength of rock-soil mixture material increases with the rock contents under low rock contents, presents obvious fluctuation under medium rock contents, and increases significantly with less fluctuation under high rock contents. The fluctuation of shear strength under medium and high rock contents is attributed to the force chain distributions affected by the spatial distribution of rock particles. The rock-soil mixture has high shear strength if stable force chains are generated. On the contrary, it has low shear strength if weak force chains are produced in the mixture. This work can be helpful in revealing inherent mechanism on micro-scale of the deformation and shear strength characteristics of soil-rock mixtures.