Abstract: The open pit mine is located in the eastern of Inner Mongolia. The water inflow from the mine is quite a lot. The supply source is mainly surface rivers. The river water penetrates into the coal seam through deep sand and pebble layer along the hidden outcrops of the coal seam, and then flows into the pit. Using the sparse well to drain the pit water causes huge waste of and serious damage to the environment, and increasing mine production cost. In order to cut off the supply channel, reduce the water inflow in the mine, and protect the water resources, the effects of fly ash on the setting time, concretion rate and strength of cement slurry were tested by in outdoor trials. At the same time, the effects of sodium silicate mixed in the cement slurry was tested. In view of the characteristics of coal seam thickness and wide supply range of River open pit mine, on-site grouting and water blocking test was carried out. The results show that the setting time is linearly positively correlated with the amount of fly ash. Sodium silicate can effectively reduce the setting time of cement fly ash slurry. There was a linear positive correlation between the setting time and sodium silicate content, and a linear negative correlation with cement content. The concretion rate was linearly positively correlated with the amount of fly ash and sodium silicate. While the strength was linearly negatively correlated with the amount of coal ash, but linearly positively correlated with the amount of sodium silicate. Mixing proper amount of sodium silicate can control the spreading distance of the slurry, increase the concretion rate and the strength. The thick coal seam grouting and water blocking test was carried out through the northwest side replenishment area of the open-pit mine. The mine water inflow was reduced by 12000 m3/d. The effect of the slurry is significant, indicating that injecting cement fly ash and cement fly ash-water glass double-liquid slurry into the cracks and voids of thick coal seams can cut off the seepage channel and reduce the water inflow in the mine.