Abstract: This is an article in the field of mining engineering. Montmorillonite is a kind of clay mineral which has significant impact on the sedimentation of coal slime water. Its charged surface and easy hydration hinder the mutual contact between particles, keeping the suspension stable dispersion and worsen the sedimentation of slime water. Based on sedimentationexperiments, we studied the effects of common metal cations Na⁺, K⁺, Mg²⁺, Ca²⁺and Al³⁺ on the sedimental characteristics of montmorillonite. Meanwhile, the mechanism of various metal cations in the hydration and sedimentation behavior of montmorillonite was investigated by using microcalorimeter, Zeta potential analyzer and infrared spectrometer. The experiment results show that, adding Al³⁺ and Ca²⁺ to montmorillonite suspension or montmorillonite-containing slime water can result into smaller sedimentation compressed layer and lower turbidity of the supernatant. So, Al³⁺ and Ca²⁺ show better performance in thesedimentation when compared with Na⁺, K⁺, Mg²⁺. The mechanism analysis shows that metal cations compress the electric double layer on the surface of montmorillonite, thus inhibit the hydration of montmorillonite, and are conducive to the agglomeration and sedimentation of montmorillonite. The research results have scientific significance for the treatment of montmorillonite-containing coal slime water.