<p>The stability of underground engineering structures is critically influenced by the mechanical behavior of backfill-surrounding rock structures. This is particularly true at their interfaces, where rough shear fracture surfaces form. Accurately describing the 3D morphological characteristics of these shear fractures is essential for both geological hazard prevention and the stability of underground engineering. To this end, the direct shear tests were conducted on backfill-rock specimens with rough interfaces. Compared to the maximum apparent dip angle (<i>θ</i> ^∗_max), the characteristic angle (<i>θ</i>^*_c) provides a more accurate representation of the relationship between the apparent dip angle and the effective contact area of shear fracture surfaces. The initial contact area ratio (<i>A</i>₀) increased as the uniaxial compressive strength of backfill (<i>UCS</i>_<i>b</i>) decreased and normal stress (<i>σ</i>_<i>n</i>) increased, which is different from the typical behavior observed in jointed rock specimens that remain stable at 0.5. By linking the 3D morphological parameters of the shear fracture surfaces to joint roughness coefficient (<i>JRC</i>), the <i>JRC</i>-Joint Compression Strength (<i>JCS</i>) model for shear strength was revised to enable accurate prediction of shear strength. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) test results show that the decrease in <i>UCS</i>_<i>b</i> and increase in <i>σ</i>_<i>n</i> reduce shear fracture surface roughness by increasing particle size and enhancing particle compaction. This study enhances our understanding of shear behavior at backfill–rock interfaces. It introduces a novel framework for describing shear strength with the backfill-rock composite structure, which is expected to offer valuable insights for improving the stability of underground engineering structures.</p>