As common backfill materials, soil and rock mixtures (S-RMs) are widely used in high-fill slope engineering projects. The shear resistance of the interphase between the S-RM and bedrock is usually weak. To improve the stability of the slope, the bedrock can be excavated into a bench-like shape. However, the shear mechanical properties of benched interphases are complex and need to be clarified. The coupling of the finite difference method (FDM) and discrete element method (DEM) creates a powerful tool for simulating soil-rock contact. In this paper, a coupled FDM-DEM is proposed to simulate the benched interphase that considered the microstructure of an S-RM and demonstrated high computational efficiency. First, the method was validated with the results of laboratory tests. Then, the typical failure characteristics of the benched interphase were simulated and the impacts of the physical parameters of the S-RMs were discussed. According to the results, the macroscopic mechanical response of the benched interphase was closely related to the changes in the skeleton structure formed by the rock blocks and benched bedrock. Consequently, the rock block rotation, force chain distribution, crack distribution, shear stress-displacement response, and strength of the interphase underwent regular changes. Overall, the influence of the rock block proportion was more significant than the influences of the rock block shape and maximum rock block size. Therefore, to improve the stability of the high-fill slopes of S-RMs, the rock block proportion should first increase, and then the rock block shape irregularity and maximum rock block size should increase.