Abstract: Impermeable grouting is an important means to solve the problem of mud and water inrush under complex geological conditions in engineering practice. The sealing effect of grouting largely depends on the basic conditions of target aquifer, the development characteristics of water channel, the selection of grouting materials and grouting technology. As a bulk industrial solid waste, fly ash has special physical and chemical properties. The utilization of fly ash as a resource can not only reduce environmental pollution, but also improve the economic benefits of fly ash. In order to master the engineering properties of fly ash-based impervious grouting materials, the basic properties of fly ash-Portland cement mixed cementitious materials were tested by orthogonal experiment method. The influence law of water-solids ratio, cement-fly ash ratio, activator and temperature on fluidity, density, concretion rate, water precipitation rate, initial setting time and final setting time of fly ash based impermeable grouting materials is analyzed by means of range analysis and variance analysis. The results show that the water-solid ratio has great influence on the six performance indexes of the slurry. The solid ratio mainly affects the fluidity and setting time of slurry. The setting rate of slurry can be accelerated by adding activator appropriately. The increase of temperature is beneficial to shorten the setting time, increase the concretion rate and decrease the water extraction rate of slurry. The optimal solution of slurry has been put forward by comprehensive equilibrium analysis method：the water-solids ratio is 0.8∶1, the cement-fly ash ratio is 30∶70, the mixing amount of activator is 2%, and the temperature should be above 15 ℃ as far as possible. The optimal scheme has been preliminarily applied in the research and demonstration project of underground curtain water interception technology in an open pit coal mine in China. The results show that a continuous and complete curtain is formed after the slurry consolidation, which fundamentally cuts off the groundwater seepage supply channel in the mining area, reduces the pressure of water transport and discharge in the mine, ensures the safety of the coal mine operation, and saves the engineering cost.