Based on washing water Ca²⁺ adsorption  on surface of clay particle to change clay particles surface features, by flotation and flocculation sedimentation experiments, divalent ion （mainly Ca²⁺） content influence to flotation and settlement of slime water system, reasonable gypsum added dose was settled. The result showed that Ca²⁺ exerted obvious effect on coal slime flotation and agglomeration, based on the actual production of the coal preparation plant, it is advisable to keep gypsum 500～650 kg and electrical conductivity 2.5～3.5.