In this paper, the phenomenon of concentrated water outflow in the mining process of No.3 coal seam in Songxinzhuang Coal Mine of Jijiajing Mining Area in Ningdong Coalfield was taken as the research object. Through whole water quality analysis, organic matter detection, three-dimensional fluorescence detection and isotope test, the characteristics of main ions, salinity and organic matter content in groundwater of roof composite aquifer and underground water outflow were found out, and the mechanism of concentrated water outflow in No.3 coal seam was obtained from the perspective of water chemistry. The results showed that the hydrochemical type of air oxidation zone water, Zhiluo Formation and Yan 'an Formation in Songxinzhuang Coal Mine was SO4·Cl-Na type before concentrated water outflow, and the hydrochemical type of goaf was Cl·SO4-Na type. After concentrated water outflow, the hydrochemical type of aquifer in Zhiluo Formation and Yan 'an Formation was Cl·SO4-Na type, the hydrochemical type of water in wind oxidation zone was SO4·Cl-Na type, and the hydrochemical type of concentrated water outflow was SO4·Cl-Na type and Cl·SO4-Na type. The formation mechanism of concentrated water in No.3 coal seam of Songxinzhuang Coal Mine was that the water of Zhiluo Formation overflows to the water of Yan 'an Formation to form mixed water samples, and the leaching occurred during the overflow process. Due to the existence of soft rock in the roof, the separation cavity was formed, and the cation reverse adsorption occurred after the separation cavity was recharged by the mixed water source. Finally, the separation cavity broke into the working face, which caused concentrated water outflow. According to isotope calculation, in the process of normal mining, the groundwater of Zhiluo Formation in the goaf water accounted for about 13%, and the groundwater of Yan 'an Formation accounted for 87%. In the process of concentrated water outflow, the groundwater recharge of Zhiluo Formation was 24%, and that of Yan 'an Formation was 76%.