Abstract: Shearing dislocation is a common failure type for rock–backfill interfaces because of backfill sedimentation and rock strata movement in backfill mining goaf. This paper designed a test method for rock–backfill shearing dislocation. Using digital image technology and three-dimensional (3D) laser morphology scanning techniques, a set of 3D models with rough joint surfaces was established. Further, the mechanical behavior of rock–backfill shearing dislocation was investigated using a direct shear test. The effects of interface roughness on the shear–displacement curve and failure characteristics of rock–backfill specimens were considered. The 3D fractal dimension, profile line joint roughness coefficient (JRC), profile line two-dimensional fractal dimension, and the surface curvature of the fractures were obtained. The correlation characterization of surface roughness was then analyzed, and the shear strength could be measured and calculated using JRC. The results showed the following: there were three failure threshold value points in rock–backfill shearing dislocation: 30%–50% displacement before the peak, 70%–90% displacement before the peak, and 100% displacement before the peak to post-peak, which could be a sign for rock–backfill shearing dislocation failure. The surface JRC could be used to judge the rock–backfill shearing dislocation failure, including post-peak sliding, uniform variations, and gradient change, corresponding to rock–backfill dislocation failure on the field site. The research reveals the damage mechanism for rock–backfill complexes based on the free joint surface, fills the gap of existing shearing theoretical systems for isomerism complexes, and provides a theoretical basis for the prevention and control of possible disasters in backfill mining.