Abstract: To solve the problems of low roof-contacted filling rate and poor roof-contacted filling effect in mine filling stope, a plastic expansive agent (PEA) was introduced into the filling slurry for active roof-contacting and roof safety. Firstly, the response surface methodology (RSM) was employed for investigating the influence laws of slurry mass concentration, cementitious powder content and PEA content on the volume change rate of cemented unclassified tailings backfill (CUTB) in this paper. Subsequently, the mechanism of PEA on CUTB was explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Finally, the expansive roof-contacted filling effect of PEA on CUTB was verified by the field industrial test. The results indicate that PEA content exerts the greatest effect on the volume change rate of CUTB, followed by slurry mass concentration, while cementitious powder content has the least influence. CUTB without PEA is consistently shrinkable at all curing ages. However, there is a critical value of PEA content, which makes CUTB with PEA convert from shrinkage characteristics to expansion characteristics. The expansion deformation mainly occurs before curing 3 d, and tends to stabilize after curing 3 d. This demonstrates that PEA can make CUTB cause expansion deformation in the plastic stage, and compensate for the early shrinkage. Microscopic analysis reveals that PEA does not alter the types of hydration products of CUTB, but promotes the formation of a large amount of expansive mineral ettringite inside the CUTB, resulting in the looseness of internal structure and the expansion of CUTB. The field measured results align relatively with test results, and the volume change rate of CUTB without PEA reaches –3.45%, while the volume change rate of CUTB with 6×10^–4 PEA content is 3.04%. The research results can provide some theoretical guidance and engineering suggestions for the roof-contacted filling technology and application of the mines.