Abstract: In order to effectively simulate the mechanical behavior of lateral flexible latex film used in large-scale triaxial tests on soil-rock mixture (SRM), a feasible three-dimensional discrete element (3-D DEM) modeling method, named 3-D wall-arrangement method for the flexible membrane boundary, is proposed. In conjunction with the previously developed 3-D DEM modeling method for irregularly shaped rock blocks and SRM, the 3-D DEM models of SRM large-scale triaxial specimens are constructed. Then, the parallel-bond model is introduced to better simulate the cementation in cemented SRM, and the meso-mechanical parameters of numerical specimens are calibrated comprehensively by conducting a series of numerical large-scale triaxial tests. Finally, the deformation and failure processes and characteristics of numerical specimens of uncemented and cemented SRMs are analyzed and compared with the results of the corresponding laboratory experiments. The results show that the proposed modeling method for three-dimensional flexible membrane boundary presents many obvious advantages including simple principle, fewer parameters, convenient implemention and saving computing resources. The stress-strain characteristics of SRM, bulging deformation of uncemented SRM, deformation localization process and meso-structural characteristics of shear band of cemented SRM are all well reproduced by carrying out the DEM simulation of large-scale triaxial test on the constructed and calibrated SRM model with the developed three-dimensional flexible membrane boundary.