Abstract: Hemihydrate phosphogypsum (HPG) based filling materials have become a new low-cost green alternative for early strength filling materials, and provide a promising solution for the large-scale utilization of phosphogypsum. However, pipe plugging, caused by the poor workability of HPG-based filling materials, has become a major safety hazard in the filling process. It is urgent to determine an economical and practicable method to improve the workability of HPG slurry work. Firstly, this work found that grinding treatment was significantly superior to increasing concentration (59% to 65%) and adding tailings (20% to 100%) in improving the HPG slurry workability based on the 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 HPG slurry workability was quantitatively described based on the micro analysis. Moreover, the direct evidence for the transformation from the robust embedded structures into the soft stacking structures is presented for the first time. In practice, it is recommended to prepare the filling materials by mixing HPG grinding for 20 minutes and 0-200% phosphorus tailings for satisfactory workability and mechanical performance. The findings of this study will provide practical and feasible methods for solving the stable transportation problem of HPG slurry.