Multistate mechanics is one of the important mechanical behaviors of granular materials. To perform an efficient calculation of the multiscale mechanics of granular materials based on the coupling of FEM and DEM, a GPU-based solver named as Coupling Simulator (CoSim) is developed. Taking sand as the example of granular materials, triaxial tests are performed to verify the reliability of the multiscale simulation. Firstly, based on the laboratory tests, a parameter inversion method is introduced to obtain the contact parameters of the particles composing the RVE of the sand sample. Then the parameters and RVE sample are used for multi-scale simulation of the triaxial test. The numerical results show that the stress-strain relationship and deformation characteristics are corresponding well with that of the laboratory tests on the macro-scale. According the numerical tests, the meso-mechanical behaviors at different positions of the sample varies greatly, but the deformation and stress evolution of the sample on macro scale are all closely related. Unlike the shear bands where both of the volume expansion and dramatic adjustment of the meso-structure take place, a numerical invalidation of Gaussian points around the shear bands is induced by the volume expansion with no or very slight adjustment of the mesostructure takes place. Finally, the strength reduction method is induced to the multi-scale algorithm and is used for the stability analysis of a sand slope. The numerical results show that both of the factor of stability (FOS) and the obtained slide face are corresponding well with that of the limit equilibrium method. So, the developed GPU-based FEM-DEM coupling code can better be used for the multi-scale simulation of granular materials and geotechnical engineering. (C) 2020 Elsevier B.V. All rights reserved.