The paper discusses the relationship between three forms of carbonate as H₂C0³, HC0₃^- and CC₃^2- and water pH value when the groundwater temperature and partial pressure of C0₂ vary so as to evaluate the reliability of water quality. It finds out that the demarcation point of pH that is called pH_O when the quantity of HCO₃^- reaches maximum, is from 7.92 to 8.60 in water and from 8.28 to 8.57 in open groundwater system of carbonate rock when pressure of CO₂ in groundwater varies from 1 to 10⁵ Pa and the groundwater temperature varies from 0 to 30℃ . pH_O will be lower and lower with groundwater temperature being higher and higher. The pH_O in groundwatar of limestone is almost equal to that of dolomite limestone owing to the solubility of dolomite is very low to calcite. So features of carbonate equilibrium in carbonate rock depend on component of calcite.