Abstract: In order to solve the problems of poor stability and low stone rate of cement fly ash slurry commonly used in cavity-typed goaf treatment engineering, a new type of grouting material for dam foundation goaf stability is made by using cement as basic material and adding bentonite. The stable slurry has low water separation rate, high stone rate and viscosity. After grouting, the permeability of the formation is reduced to meet the anti-seepage requirements of the project. In this paper, the laboratory physical and mechanical properties of grout with different mix ratios are tested and analyzed, and the grouting effect of the selected stable grout is selected through the formation permeability evaluation before and after grouting. The results show that the stability, stone formation rate and viscosity of the slurry can be improved significantly by adding bentonite, but the fluidity of the slurry can be reduced at the same time. With the increase of bentonite content, the initial setting time and final setting time increase, and the strength of the slurry stone body decreases. The formation permeability before and after grouting is analyzed and compared by the packer test, and the grouting effect of the selected stable slurry (water cement ratio 0.8∶1, bentonite content 5%) is verified to meet the engineering design requirements.