<p>The soil-rock mixture composed of cutting and tunnel excavation waste is widely used in the construction of highway subgrade in mountainous areas. The particle size of earth-rock mixtures varies significantly, and the shear properties exhibit a particle size effect. Test results obtained from specimens with different particle sizes are not entirely representative, making it difficult to apply them to engineering practice. In this paper, a large-scale direct shear test of soil-rock mixture with different particle sizes and water contents under the condition of fixed stone content was carried out. The effects of particle size and water content on the shear stress-strain relationship and shear strength of soil-rock mixture were studied, and the interaction between water content and particle size was discussed. Furthermore, the effect of the water content and the particle size on the stability of the embankment is studied in detail based on numerical limit analysis method. It is shown that: 1) The shear stress-strain relationship of soil-rock mixture has two forms: strain hardening and strain softening. 2) The shear strength of the mixture decreases with the increase of water content. When the filler is dry, the decrease is more obvious. The strength parameters decrease with the increase of water content, and the cohesion is more affected by water content. 3) With the increase of particle size, the internal friction angle of the mixture increases, the cohesion decreases, and the overall shear strength increases. The effect of particle size on the mechanical properties of the mixture is weakened at high water content. 4) The collapse mechanism is altered by the particle size, which changes from global failure to surface failure gradually with the increase of the particle size.</p>