Abstract: The calculation results of water quality monitoring of the acid mine drainage circulation system of Shandi river in Yangquan City, Shanxi Province for many years show that the Ca/Mg value of the acid goaf water in coal mine drainage is generally low. The average value of 257 groups of acid mine drainage samples was 1.14, the average value of 69 samples of mine drainage in the study area during the same period was 3.73, and the average value of 206 samples of karst groundwater was 5.30, and there is a law that the Ca/Mg value decreased with the increase of acidification degree (increases of SO₄²⁻ content or decreases of pH value). In response to this problem,combined with the geochemical source conditions of the study area, this paper analyzes the relationship between the saturation index of gypsum, calcite and dolomite minerals and pH value changes in laboratory tests and field monitoring water samples, so as to reveal the genetic mechanism of low Ca/Mg values of acid goaf water in coal mine drainage. The research results show that the scattered magnesite and pyrite in the carbonate interlayers in the Carboniferous-Permian coal measure strata in the study area are the material sources of Ca²⁺, Mg²⁺ and SO₄²⁻ in the goaf water; In the sulfate-dominated acidic solution formed by the oxidative hydrolysis of pyrite (pH value is in the range of 2.0 to 4.5), the saturation index representing the solubility of sulfuric acid to gypsum, calcite and dolomite is in the order of gypsum>calcite>dolomite,at the same time, with the increase of pH value, gypsum will occur in the precipitation reaction, the relative content of Ca²⁺ would decrease, and the relative content of Mg²⁺ would increase, which will eventually lead to the decrease of Ca/Mg value in the solution. Its chemical mechanism is, (1) When the pH value is in the range of 2 to 3, the dissolution of gypsum is inhibited by high concentration sulfuric acid, the solubility decreased rapidly with the increase of pH value, gypsum precipitation may occur,which have been confirmed in laboratory tests, and the content of Ca²⁺ decreased;When the pH value is in the range of 3 to 4.5, the gypsum in the samples is in a reaction equilibrium state with a slight change in the saturation index near zero. A slight increase in pH value will lead to the formation of gypsum precipitation from Ca²⁺ dissolved from calcite and dolomite. The content of Ca²⁺ cannot continue to increase, and the relative content of Mg²⁺ increases. (2) In the pH range of 2 to 4.5, the calcite is affected by the co-ion effect of Ca²⁺ and the ion-pair balance when the gypsum is saturated, the saturation index of all samples was maintained at about −4.4, irrespective of pH decrease. (3) However, the dissolution of dolomite was not affected by the above inhibition (the solubility of MgSO₄ in sulfuric acid solution is greater than that of CaSO₄), the Mg²⁺ and SO₄²⁻ content of the monitored samples have a significant positive correlation (the linear correlation coefficient reached 0.83). While dolomite forms gypsum precipitation with Ca²⁺ in the solution, de-dolomitization reaction occured sustainably, and there is dissolution of magnesite, which eventually increased the content of Mg²⁺ in acid water relative to the content of Ca²⁺, and the Ca/Mg value decreased. The Ca/Mg value can be used as an indicator of pollution characteristics of acid goaf water in coal mine drainage and applied to environmental impact assessment.