Abstract: Montmorillonite is the main factor affecting the settling performance of coal slime water,static settling tests and pressure filtration experiments were designed to study inhibition of potassium ions on hydration swelling of mont- morillonite. Volume expansion of montmorillonite in deionized water is 2. 3 times greater than the original volume of dry powder,while the volume expansion of montmorillonite in the solution of 0. 1 mol / L potassium ions is only 1. 8 times greater than the original volume. The filtering speed of montmorillonite in the solution of potassium ions is obvi- ously faster than that in deionized water. Hydration swelling of montmorillonite could be inhibited by potassium ions, and Inhibitive effect of potassium ions on hydration swelling of montmorillonite is better than calcium ions. The inhibi- tive mechanism of potassium ions on hydration swelling of montmorillonite is that the potassium ion was adsorbed on montmorillonite interlayer,potassium ion is not easy to hydration,the water molecules between the crystal layer is dis- charged,and the bonding layer between the potassium ions greatly restrain the expansion or separation process of adja- cent crystal layers.