The contact surface between the coarse-grained soil and the stainless-steel blank was tested by an improved direct shear apparatus. Four relative compactness, which is 40%, 55%, 70% and 85%, were used for the coarse-grained soil. The contact mechanical properties of the interface between coarse-grained soil and steel plate and the influence of relative density of coarse-grained soil are studied. The results show that before the interface between coarse-grained soil and steel plate reaches shear failure, the relationship between shear stress and shear displacement is approximately linear. The shear strength and shear stiffness coefficient of the contact surface increase with the increase of relative density of coarse-grained soil. When the relative density increases from 40% to 85%, the cohesion of the contact surface increases by 41. 8% on average, the internal friction angle increases by 35. 4% on average, and the shear stiffness coefficient k_s increases by 11. 2% on average. The tangential stiffness coefficient k_s increases by 40. 3% on average when the normal stress σ_n of contact surface increases from 100 kPa to 800 kPa. Based on the test results, the relationship between the tangential stiffness coefficient k_s, the normal stress σ_n and the relative density D_r is found, and an empirical formula of k_s-σ_n-D_r is proposed, thus a simple contact-surface constitutive model is established.