Abstract: The mechanical properties of cemented paste backfill (CPB) determine its control effect on the goaf roof. In this study, the mechanical strength of polymer-modified cemented paste backfill (PCPB) samples was tested by uniaxial compression tests, and the failure characteristics of PCPB under the compression were analyzed. Besides, acoustic emission (AE) technology was used to monitor and record the cracking process of the PCPB sample with a curing age of 28 d, and two AE indexes (rise angle and average frequency) were used to classify the failure modes of samples under different loading processes. The results show that waterborne epoxy resin can significantly enhance the mechanical strength of PCPB samples (when the mass ratio of polymer to powder material is 0.30, the strength of PCPB samples with a curing age of 28 d is increased by 102.6%); with the increase of polymer content, the mechanical strength of PCPB samples is improved significantly in the early and middle period of curing. Under uniaxial load, the macro cracks of PCPB samples are mostly generated along the axial direction, the main crack runs through the sample, and a large number of small cracks are distributed around the main crack. The AE response of PCPB samples during the whole loading process can be divided into four periods: quiet period, slow growth period, rapid growth period, and remission period, corresponding to the micro-pore compaction stage, elastic deformation stage, plastic deformation stage, and failure instability stage of the stress–strain curve. The AE events are mainly concentrated in the plastic deformation stage; both shear failure and tensile failure occur in the above four stages, while tensile failure is dominant for PCPB samples. This study provides a reference for the safety of coal pillar recovery in pillar goaf.