Abstract: The shear characteristics of the interface formed between a cemented tailings backfill (CTB) and surrounding rocks play a crucial role in the design and stability of underground goafs. To investigate the shear behavior of CTB–rock interfaces, rock samples representing the topography of surrounding rocks were constructed using 3D morphology scanning and engraving techniques. A series of direct shear tests were conducted on the CTB rock samples to examine the influence of the cement–tailings ratio on the interfacial shear behavior. The results showed that the compressive strength of the CTB and shear strength of the CTB–rock interface decreased with decreasing cement proportion. With deceasing cement content, the failure area of the CTB after the test increased, and the roughness of the newly generated interface reduced. A digital image correlation analysis revealed that the compressive stress concentration in the region with an obtuse angle with respect to the shear direction was the primary cause of CTB failure. Moreover, the correlation between the wear area and the silicon-dense area helped confirm that the silicon particles are more prone to failure in these areas than in other regions. Our findings provide new insights into the shear sliding mechanism at CTB–rock interfaces and can aid in the selection of the cement–tailings ratio at engineering sites. For example, if the horizontal principal stress of the surrounding rock mass in a backfilling area is relatively high, the cement content can be reduced for CTB applications.