Abstract: In order to study the shear characteristics of open-pit slope mudstone under the condition of flooding, direct shear tests are carried out on three kinds of mudstone under different normal stress conditions in the dry and saturated state of Shenshan open-pit in Inner Mongolia. The results show that the macroscopic shear mechanical characteristics of the same mudstone under different test conditions and different mudstones under the same test conditions are different. The osmotic effect of mudstone immersed in water is the main mechanical factor affecting its shear properties. The influence of flooding conditions on slope stability and instability mode of Shenshan open-pit mine is studied by numerical simulation. The research results have important reference value for the safety production of Shenshan open-pit mine and similar mines.