Abstract: Based on the water test data of No.15 coal roof K₂ limestone and No.15 coalbed methane well in the southern Qinshui basin, the characteristics, types and the genetic mechanism of water chemical composition were systematically studied by statistical and comparative methods. The evolution model was built to systematically describe the various reactions and effects of limestone and coalbed water in the recharge zone, runoff zone and detention zone. The results show that the K₂ limestone water and the No.15 coal seam water will undergo ion exchange, mixing and CO₂ action. The K₂ limestone water is mainly Ca-Mg-HCO₃-SO₄ type in the recharge zone and the strong runoff zone, and the water type of the coal seam is Na-HCO₃-SO₄ type; K₂ limestone water in slow-flow area is generally Na-Ca-Cl-HCO₃ type, while coalbed water is mainly Na-Cl-HCO₃ or Na-HCO₃ type; K₂ limestone water is the same as the coal seam water in detention area, and it is Na-HCO₃ or Na-Cl-HCO₃ water type. The study aims to provide a reliable hydrogeological basis for the development of No.15 coalbed methane.