Abstract: Groundwater resources are destroyed seriously during the process of coal mining activities, which is difficult to solve throughout the green coal mining. The coal mine underground reservoir effectively solves the problems of mine drainage protection and water resource recycling. In order to clarify the mechanism of water purification in coal mine underground reservoir, this paper analyzed the water hydrochemical characteristics and formation mechanism of underground reservoirs, based on the determination of water quality index of in-situ incoming and outgoing water samples in Shendong Daliuta Coal Mine, the combination of correlation analysis, principal component analysis and other multivariate statistical methods with Schukalev classification, Piper trigram and other hydrochemical analysis methods and ion ratio method. The results show that the self-purification effect of underground reservoir in coal mine is significant. The indicators of SS, NTU, EC and TDS are significantly reduced. The average removal rates of COD, Fe3+ and Mn2+ reach 42%, 89% and 94% respectively. The hydrochemical type is gradually changed from Ca2+·Na+ - Cl-·SO2-4 type of influent to Na+·Ca2+- Cl-·SO2-4 type of effluent. Cation exchange occurs in the process of reservoir purification. The results of multivariate statistical analysis show that filtration, mixing, external interference and cation exchange affect the purification process of coal mine underground reservoir water quality, among which filtration plays a leading role. Na+, Ca2+ and Mg2+ mainly come from the dissolution of silicate minerals.