Abstract: For the challenges of low roof-contacted rates and poor filling performance in mine backfill stopes, a plastic expansive agent (PEA) into the filling slurry was introduced to enhance roof-contacting efficiency and stability. The response surface methodology (RSM) was employed to investigate the effects of slurry mass concentration, cementitious powder content, and PEA content on the volume change rate of cemented unclassified tailings backfill (CUTB). The mechanism of PEA on CUTB was further analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM), and its expansive roof-contacting effects were validated through field industrial testing. The results indicate that PEA content has the most significant influence on the volume change rate of CUTB, followed by slurry mass concentration, with cementitious powder content having the least impact. CUTB without PEA exhibited consistent shrinkage across all curing ages. However, a critical PEA content threshold was identified, beyond which the behavior of CUTB shifted from shrinkage to expansion. Expansion deformation was primarily observed within the first three days of curing and stabilized thereafter. This expansion, induced by PEA during the plastic stage, effectively compensates for early-stage shrinkage. Microscopic analysis revealed that PEA does not alter the types of hydration products in CUTB, but promotes the formation of expansive mineral ettringite, resulting in a looser internal structure and volume expansion. Field measurements corroborated the experimental findings: CUTB without PEA exhibited a volume change rate of −3.45%, while CUTB with a PEA content of 6×10⁻⁴ achieved a positive volume change rate of 3.04%. These findings provide theoretical insights and practical recommendations for improving roof-contacting filling technologies in mining applications.