The mechanical response of a soil-rock mixture (SRM) is crucial in railway, airport, highway, and dam engineering. However, existing studies of the SRM often fail to account for the effect of its components (e.g., fine content) and particle crushing. This paper aims to develop a thermodynamically consistent constitutive model that can simulate the mechanical behaviour of the SRM. Based on the initial fine content (FC₀), we quantify the components of the SRM and explore the effect of FC₀ on the mixture’s mechanical responses. The FC₀-related energy dissipation due to particle crushing is highlighted in the proposed model. The model is validated by three experimental cases with different FC₀, which shows that our model satisfactorily captures the complex SRM behaviours with only eight physically identifiable parameters. The model also confirms the experimental phenomenon that the FC₀ has a substantial effect on both the shear strength and energy dissipative ratio of soil-rock mixtures.