Abstract: In this article,the relationship between the three existing forms of carbonate and pH value is studied at different temperatures and CO₂ pressures in an open underground water system while activity is taken into consideration.We find that the value of pH₀ decreases with the increase of temperature and tends to a stable value at a given CO₂ pressure and the value of pH₀ decreases with the increase of the CO₂ pressure at a given temperature when CO₂ pressure changes in the range of 10～10⁵Pa and the temperature changes in the range of 0～30℃.However,no matter how the temperature and CO₂ pressure of the underground water change,pH₀=7.92～8.60.These conclusions reflects the actual conditions of the underground water and can be better used for the review of water analysis data.