Abstract: The silicate-sulphoaluminate cement (PO−SAC) composite slurry has the advantages of short setting time, high early strength and good impermeability, and has broad application prospects in the treatment of coal mine water inrush. In the process of actual grouting parameter design, it is necessary to reveal the rheological characteristics and viscosity time-varying law of PO−SAC composite slurry. The effects of compound ratio and water-cement ratio on the viscosity time-varying behavior of PO−SAC composite slurry were studied. The composite ratio of the slurry is from 10∶0 to 0∶10, and the water-cement ratio is from 0.5 to 1.0. The results show that Portland cement (PO) and sulphoaluminate cement (SAC) have a hydration superposition effect, and 30% SAC has the strongest promoting effect on OPC, which increases the slurry viscosity by more than 4 times. The water-cement ratio of the composite slurry is inversely proportional to the viscosity value and the viscosity growth rate.The time-varying curve of the viscosity of the composite slurry under different water-cement ratio conditions conforms to the form of a quadratic equation. The flow pattern of the slurry is related to water-cement ratio and has nothing to do with the composite ratio. The yield stress and plastic viscosity of the slurry decrease with the increase of water-cement ratio. The water reducing agent can significantly reduce the initial viscosity of the composite slurry, and the initial viscosity of the sample is reduced by more than 82.46% when the water reducing agent is 1‰. The PO−SAC material was applied to a coal mine floor water inrush treatment project, which effectively blocked the aquifer fissure water and ensured the safe mining of coal mines. It can provide reference for the establishment of PO−SAC slurry grouting theory, numerical calculation and grouting parameter design, and help the efficient treatment of water inrush in coal mines.