The pile-rock(soil) interface mechanical behavior was the basis of pile foundation bearing mechanics. Firstly, the mechanical property of the interface was investigated through the laboratory large direct shear test of red mudstone pile-rock interface. The results indicated that the shear stress-strain curve presented strain softening behavior. The shear stress was increasing with the shear strain initially, and when the shear stress came to a peak, the shear stress was decreasing with the shear strain and finally reached a steady. Subsequently, the constitutive equation was deduced based on the interface mechanical property, the ideal elastic-plastic contact element in FLAC^3D was further developed through fish language. Additionally, the direct shear test of red mudstone was simulated according to the interface constitutive model, and the relationship of shear stress and shear displacement between the pile and soil were analyzed which proved that the constitutive model showed good performance on simulating strain softening features of the interface.