Abstract: Hemihydrate phosphogypsum (HPG)-based filling materials have become a new low-cost green alternative for early strength filling materials. They also provide a promising solution for the large-scale utilization of phosphogypsum. However, pipe plugging, which is caused by the poor workability of HPG-based filling materials, has become a major safety hazard in the filling process. Determining an economical and practicable method is urgently needed to improve the workability of HPG slurry work. First, this work found that grinding treatment was much more effective than increasing concentration (59wt%–65wt%) and adding tailings (20wt%–100wt%) in enhancing the workability of HPG slurry based on a comprehensive analysis of water retention, fluidity, and flow stability. Then, the combined effects of particle size, particle morphology, water film, and interparticle interactions on the workability of HPG slurry were quantitatively described through a microanalysis. Moreover, the first direct evidence for the transformation from robust embedded structures to soft stacking structures was presented. In practice, the filling materials should be prepared by grinding HPG for 20 min and mixing with 0wt%–200wt% phosphorus tailings to achieve satisfactory workability and mechanical performance. The results of this study provide practical and feasible methods for addressing the stable transportation problem of HPG slurry.