This study aims to investigate the impact of the stress–strain behaviour of cemented tailings backfill (CTB) on the stability of sill mat structures used in underhand cut-and-fill mining. A three-dimensional finite element model is established to simulate four types of sill mats with varying stress–strain relationships, namely ductile, intermediate ductile, intermediate brittle, and brittle. The results indicate that the stress–strain behaviour does not affect the damage evolution process of the sill mat but impacts its deformation and energy absorption capacity. The brittle sill mat exhibits severe damage and deformation, with a lower energy absorption ability than the ductile sill mat. Statistical analysis also suggests that the ductile sill mat is better at reducing damage inside the surrounding rock and has a higher structural strength than the brittle sill mat. Therefore, the study highlights the importance of considering the stress–strain behaviour of CTB in designing safe sill mat structures and suggests the potential benefits of using fibre-reinforced backfill.