The spatiotemporal evolution of multiple key layers overlying strata in thick coal seam has always been one of the key and difficult problems to be solved in the field of stratified filling, and the confined compression characteristics of fly ash slurry are the main influencing factors. In this paper, the lateral compression and viscosity test of filling materials were carried out, and the shear stress and plastic viscosity of filling materials were studied. The results show that the strain of fly ash in the initial stage of compaction accounts for 64.8% of the final strain. When the fly ash slurry is pressed, the stress increases exponentially with the strain, and the volume weight has a logarithmic relationship with the compaction stress. Both shear stress and plastic viscosity decrease negatively with the increase of water-cement ratio. When the ratio of fly ash to water to cement is controlled between 1.2 and 1.4 and the mass concentration is controlled between 41.7% and 45.5%, the slurry fluidity, pumpability and grouting efficiency are high.