Abstract: SouthtoNorth water diversion will not only be used for supply after mixed with groundwater in Beijing, but also injected underground for save. The change in the mixed water quality is focused in this paper. Water in the Danjiangkou reservoir and groundwater in Beijing were sampled and mixed experiments were carried out. The results show that sodium, magnesium, chloride and sulfate are mixed physically and calcium, bicarbonate and pH are drastically affected by carbonate balance. When the ratio of water in the Danjiangkou reservoir to groundwater in Beijing is large, the nitrification dominates the carbonate balance. On the contrary, the change in carbon dioxide pressure affects the carbonate balance. The research results are useful to water supply for the SouthtoNorth water diversion.