The primary objective of this study is to investigate the trends of water quality in coproduced water over time. Forty-six water samples were collected over the drainage time from 2 to 2028 d from 12 coalbed methane wells in the Beier mining area of the Daxing Mine in the Tiefa Basin, which is a continental deposit in China. Nine ions, including chloride, nitrate, sulfate, bicarbonate, carbonate, potassium, sodium, calcium, and magnesium, were tested using an Ion-chromatograph ICS-1500. The results indicate that the water quality of coproduced water has a close relationship with the drainage time. The variations of water quality are distinguished into three stages relative to the drainage time, wherein the major ions contributed to the TDS are different in each stage. The water contains Na-Cl in the first stage, after which the water type transforms to Na-HCO3-Cl and finally turns into Na-HCO3. Higher concentrations of chloride, sulfate, calcium, and magnesium, are the representative characteristics of the first stage, which are consequences of the impact of the fracturing fluid. The second stage is characterized by the decline of chloride, sulfate, calcium, and magnesium concentrations and an increase of bicarbonate and carbonate proportion in TDS. As the drainage time increases, the major components of the TDS in the third stage change to bicarbonate, sodium, and carbonate from the second stage ions of bicarbonate, sodium, chloride, and the variation of ionic concentration tends toward stability in the third stage. After long drainage periods, the water quality of coproduced water tends to be similar to that of the original formation water.