Abstract: In order to explore the properties of the mixed slurry of fly ash and gangue, fly ash and gangue of different particle sizes were used as the test materials, the orthogonal experimental method was adopted, and the test data were processed by SPSS software to analyze the influence of volume concentration, gangue particle size and fly ash and gangue mass on the density, viscosity and water evolution rate of the slurry. Combined with the actual grouting site, the viscosity reduction test is designed. The results showed that the three factors of volume concentration, gangue particle size, fly ash and gangue mass ratio all played a significant role in the change of slurry density and slurry viscosity. Among them, gangue quality accounts for 70% of the quality will make the viscosity of the slurry increase faster; the volume concentration and the quality of fly ash and coal gangue play a major role in the change of slurry water evolution rate, of which the quality of coal gangue in different proportions has differences in the speed of change of water evolution rate. Gangue particle size plays a secondary role in the change of slurry water evolution rate; the increase in the content of polyhydroxylic acid superplasticizer has a significant effect on the reduction of viscosity of fly ash slurry and fly ash gangue mixed slurry, and gradually slows down to the extreme value with the increase of the dose.