Abstract: For mines with poor ore bodies and surrounding rocks, the general mining method does not allow the ore to be extracted from underground safely and efficiently. For these mines, the downward layered filling mining technique is undoubtedly the most suitable mining method. The downward filling mining technique may eliminate the troubles relating to poor ore deposit conditions, such as production safety, ore loss rate, and depletion rate. However, in this technique, the safety of the artificial roof of the next stratum is of paramount importance. Cementitious tailings backfilling (CTB) that is not sufficiently cemented and causes collapses could threaten ore production. This paper explores a diamond-shaped composite structure to mimic the stability of a glued false roof in an actual infill mine based on the recently emerged three-dimensional (3D) printing technology. Experimental means such as three-point bending and digital image correlation (DIC) techniques were used to explore the flexural characteristics of 3D construction specimens and CTB combinations with different cement/tailings weight ratios at diverse layer heights. The results show that the 3D structure with a 14-mm ply height and CTB has strong flexural characteristics, with a maximum deflection value of 30.1 mm, while the 3D-printed rhomboid polymer (3D-PRP) structure with a 26-mm ply height is slightly worse in terms of flexural strength characteristics, but it has a higher maximum flexural strength of 2.83 MPa. A combination of 3D structure and CTB has more unique mechanical properties than CTB itself. This research work offers practical knowledge on the artificial roof performance of the downward layered filling mining technique and builds a scientific knowledge base regarding the successful application of CTB material in mines.