Abstract: The soil-rock mixture widely used in high-fill projects is characterized by significant heterogeneity and discontinuity. The engineering characteristics of the mixture depend on the mechanical properties of soil and stone and closely relate to the relative proportion of earth and stone, particle composition and other factors. Based on the PFC^2D discrete element numerical simulation, the change rules of shear strength of the soil-rock mixture with missing intermediate particle size under different opening widths are analyzed, and the force transfer path, strength formation mechanism and deformation and failure mechanism of the mixture skeleton structure with missing intermediate particle size are deeply disscussed. The results show that the width of the opening between the upper and lower shear boxes has an important influence on the shear strength. The peak strength of the earth-rock mixture decreases with the increase of the opening width, and the shear strength-horizontal displacement curve shows strain softening. The opening width also has a significant influence on the skeleton composition of the earth-rock mixture. The larger the opening is, the less the number of the skeleton particles is, the larger the distribution range of skeleton particles is, the closer the shear band is to the upper shear box. Although the distribution of shear band is different under different opening widths, the distribution of normal contact force of particles on the skeleton is basically uniform, which shows that the stress of particles on the skeleton is relatively uniform, and there is no obvious relationship between the stress of particles on the skeleton and the opening width. Under different opening widths, the shear plane is not a plane at each shear stage. With the increase of horizontal shear displacement, the distribution of contact force chain and shear band on the skeleton also changes constantly, and the stress inside the samples is extremely uneven. The phenomenon of void inside the samples is related to the skeleton composition, friction coefficient of particles and gradation of the earth-rock mixture, and the opening width also has an important influence on them. The wider the opening, the more obvious the void phenomenon of the sample during the shear process.