Abstract: The soil-rock mixture (S-RM) is a discontinuous, heterogeneous and anisotropic multiphase medium consisting of soil and rock blocks. Its mechanical characteristics are extremely complicated and closely related to the internal mesoscopic structure of soil and rock blocks. From the view of meso-structure, a reconstruction method by computer random simulation is proposed to reconstruct the 3D meso-structure for S-RM based on irregular rock blocks, and a randomly modelling system (RMS^3D) is developed using the FORTRAN language. On this basis, the discrete element models for S-RM are established considering the shape of irregular rock blocks, and 3D numerical shear tests by particle flow code are conducted for them to investigate the influences of spatial distribution of rock blocks on the mechanical characteristics of S-RM. The results show that the mechanical characteristics are significantly affected by the spatial distribution of rock blocks. The shear stress-displacement and normal displacement-displacement curves are different for S-RM samples with the same content and gradation but different spatial distributions of rock blocks, especially for the post-peak curves, they show obvious differences between each other, and the latter lags behind the occurrence of differences compared to the former. In addition, the shapes and thicknesses of shear zones after failure exhibit certain differences for S-RM samples with different spatial distributions of rock blocks because of the effect of obstruction of the rock blocks located on the shear surface.