To investigate the effect law of surface roughness and soil relative density on the mechanical properties of sand-concrete pile interface, a series of sand-concrete interface tests with various surface roughness and soil relative densities were conducted by the large-scale direct shear apparatus. This study investigated the influence of roughness and soil relative density on shear stress-horizontal displacement, peak shear strength, secant friction angle, and normalized friction coefficient. The results showed that in the case of the smooth interface, the shear stress-horizontal displacement curves of the dense sand sample illustrated a slight softening response, and softening response became remarkable as the roughness increased. The curves of the loose sand sample showed a hardening response. Moreover，the peak shear strength of the interface increased nonlinearly with the increase of normal stress. And the greater the soil relative density，the more obvious the nonlinearity. The secant friction angle of the interface decayed exponentially with increasing normal stress. However, the peak friction coefficient decreased as a power function with the increasing normal stress due to the smaller shear strength increment. Additionally, there existed a critical value of roughness Icr，such that the peak friction coefficient and normalized secant friction angle no longer increased as the roughness increased and turned to decrease when I exceeded Icr.