Abstract: The coal mine underground reservoir has a certain purifying and removing hardness(calcium and magnesium ions) function for mine water, providing the possibility of direct underground reuse of mine water.However, the migration and transformation mechanisms of calcium and magnesium ions in the water of underground reservoirs are still unclear, which hinders the research of large-scale, low-cost treatment and reuse technologies for mine water in underground.Therefore, this research takes the Shendong Daliuta coal mine underground reservoir as the object, and combines on-site sampling, indoor experiments, and molecular dynamics simulations to investigate the mass concentration changes of Ca²⁺ and Mg²⁺ in water under different water-rock/mineral interaction conditions, revealing the migration and transformation mechanisms of Ca²⁺ and Mg²⁺ in underground reservoir.The research results indicate that: The main types of water-rock interactions in coal mine underground reservoir include leaching and adsorption, and the influences of internal rock on the migration and transformation laws of Ca²⁺ and Mg²⁺ in mine water are mainly related to the composition and quality quotient of minerals, physical and chemical properties of minerals, initial mass concentration of ions and other factors.When the initial mass concentration of ions in mine water is low, the leaching effect of soluble minerals is the dominant factor, leading to mass concentration of ions increase in mine water;as the reaction continues, the adsorption and ion exchange of clay minerals gradually become the dominant factors, causing the Ca²⁺ and Mg²⁺ mass concentration in mine water to gradually decrease and eventually reach stable equilibrium state. Kaolinite and chlorite in collapsed rocks have strong adsorption capacities for Ca²⁺ and Mg²⁺ in mine water,Kaolinite respectively has adsorption capacities of 3.35 mg/kg and 1.27 mg/kg for Ca²⁺ and Mg²⁺, while chlorite respectively has adsorption capacities of 3.39 mg/kg and 2.23 mg/kg for Ca²⁺ and Mg²⁺. Based on the theories of molecular dynamics and quantum chemistry, stable adsorption configurations of Ca²⁺ and Mg²⁺ on the kaolinite and chlorite surfaces were constructed, revealing the migration and transformation mechanisms of Ca²⁺ and Mg²⁺ on mineral surfaces, finding that the adsorption strength of Mg²⁺ is greater than Ca²⁺, and the adsorption strength of kaolinite on Ca²⁺ and Mg²⁺ is greater than chlorite.The research results provide important theoretical basis for the development of in-situ hardening treatment technology for coal mine underground reservoirs.