Abstract: In order to explain the variation law of ions in the water body of underground reservoir of coal mine,clarify its self-purification mechanism,and provide a theoretical basis for its construction and development,based on the anal- ysis of water quality,hydrochemical types and ion sources of in-situ water samples from Daliuta coal mine underground reservoir in Shendong,four groups of simulation experiments of water-rock interaction of mudstone and fine sandstone with deionized water and mine water was designed,and the variation law and source of main ions in the water body of underground reservoir were revealed by graphic method and ion ratio method. The results showed that the main water- rock interactions in the underground reservoir of Daliuta coal mine were cation exchange reaction,pyrite oxidation,dissolution of calcite, dolomite and silicate minerals. Cation exchange reaction increased Na+ concentration while de- creased Ca2+ concentration,and the hydrochemical type changed from SO4 ·Cl- Ca type of influent to SO4 ·Cl- Na type of effluent which caused the concentration of Na+ higher than that of Cl- and the concentration of r(Ca2+ +Mg2+ ) lower than that of r( HCO- +SO24- ). With the water-rock interaction proceeding,the sodium adsorption ratio of mud- stone and fine sandstone increased and the cation exchange reaction was weakened gradually. Part of SO24- in the water body of underground reservoir of coal mine was from pyrite oxidation,in which the calcite content of mudstone was more than dolomite content,mainly pyrite oxidation and neutralization reaction with calcite,while fine sandstone had more dolomite,mainly pyrite oxidation and neutralization reaction with dolomite. Na+ mainly came from the dissolution of silicate minerals such as albite and orthoclase,and the main sources of Ca2+ and Mg2+ in mudstone and fine sand- stone formations were the dissolution of calcite and dolomite,respectively.