To reveal the shear mechanism of the zigzag sandstone-concrete interface under constant normal stiffness （CNS） conditions from micro-scale and macro-scale aspects， several direct shear tests for the sandstone-concrete interface were conducted by self-transformation CNS shear apparatus. On this basis， discrete element numerical models simulating laboratory experiments were established by using the rigid wall substitution method. In addition， the motion of the sandstone sample was controlled by three explicit spring-based kinetic equations to ensure the dynamic balance of the system at each time step and realize the CNS loading. The rationality of the models was verified by comparing the numerical simulation and experimental results. Subsequently， 16 cases of numerical shear tests were conducted to further reveal the failure mode and load transfer mechanism of the interface from the micro-scale aspect by observing the micro-crack propagation and force chain evolution， and the effects of asperity geometries （i.e.， half chord-length λ， inclination θ） and boundary conditions （i.e.， initial normal stress σn0， normal stiffness K） on shear strength and dilation were analyzed from the macro-scale aspect. The results indicated that micro-cracks gradually propagated from the interface area to the interior of the rock in a trend of stable increase-rapid increase-decrease in growth rate， and the failure mode of the interface transitioned from wear to shear failure with increasing inclination. The shear strength increased as an exponential function with increasing λ， θ， σn0， and K.