Abstract: The EPS-mixed soils are composed of two solid phases (cemented soils and EPS beads) with the unique mesoscopic structure. The macroscopic behaviour of the EPS-mixed soils has been widely investigated so far, but the focus has seldom been put on the mesoscopic behaviour. In this study, following the frameworks of Mohr-Coulomb model and Drucker-Prager model respectively, the constitutive descriptions of the cemented soils and the EPS materials are developed based on their mechanical test results. Besides, the strain hardening/softening laws of the cemented soil-EPS material interface are summarized based on the interface shear tests. The refined numerical simulations of triaxial shear tests on the EPS-mixed soils are carried out, with which the macroscopic stress-strain behavior and deformation modes of the EPS-mixed soil specimens are replicated. The refined numerical simulations reveal that the three types of deformation modes of the specimens (shear banding, local lateral expansion, overall uniform deformation) can be attributed to the non-uniform mesoscopic mechanical responses. The distinct mechanical behavior of the cemented soils and the EPS materials is the origin of non-uniform stress and strain distributions, and such non-uniformity is enhanced by the non-uniform spatial distribution of the EPS beads. The two factors collaboratively determine the non-uniformity of the macroscopic deformation observed for the EPS-mixed soil specimen.