Abstract: Soil-rock mixtures are heterogeneous materials composed of coarse rocks with high strength and fine filling soils. The plasticity behavior of soil-rock mixtures is closely dependent on fine content. When the fine content is low, the soil-rock mixture is a rock-dominated structure and the plasticity behavior of soil-rock mixture is primarily controlled by the coarse grains. While the fine content is high, the soil-rock mixture is a soil-dominated structure and the plasticity behavior of soil-rock mixture is primarily controlled by the fine grains. However, the effect of fine content on the plasticity behavior of soil-rock mixtures and its mechanism remain unclear. This manuscript investigates the instability and non-associated behavior of rock-dominated soil-rock mixtures with different fine contents based on second order theory. In addition, the mesoscopic mechanism on how fine content affects plasticity behavior of soil-rock mixtures is revealed. It is found that fine grains help to stabilize the granular assembly by limiting macroscopic plastic deformations. Macroscopic plastic deformations decrease with the increase of fine content of soil-rock mixtures compared at the same stress ratio. The fine content is found to greatly affect the flow direction of soil-rock mixtures (i.e. normal direction of plastic potential surface). With the increase of fine content, the angle between normal direction of yield surface and plastic potential surface decreases. It means that the non-associated behavior becomes less pronounced with the increase of fine content. It is also found that the bifurcation domain of soil-rock mixtures becomes narrower when fine content increases. In spite of the fact that some fine grains act as skeleton grains together with coarse grains, fine grains are found not to influence the internal mechanical state of soil-rock mixtures. As a result, fine content does not change the normal direction of yield surface. Those conclusions drawn from this manuscript is of great significance to build elasto-plastic constitutive models for rock-dominated soil-rock mixtures considering the effect of fine content.