Abstract: The hydrochemical properties of five points were monthly monitored in Jiangjia Spring from from December 2017 to November 2018, in order to reveal the effect on hydrochemistry of karst spring water and karst carbon cycle from photosynthesis of aquatic plants and gas exchange. The results showed that the Ca ²⁺ concentration and HCO ₃^- concentration at JJQ3 were significantly low in the months with less precipitation, comparing with those at JJQ1 and JJQ2. Meanwhile the dissolved oxygen (DO) negatively correlated with conductivity (EC), Ca ²⁺ and HCO₃^- concentrations. It indicated that HCO₃^- generated from carbonate dissolution was partly converted into stable organic carbon through photosynthesis of aquatic plants, which enhanced the karst carbon sink. However, the high flow rate could limit the photosynthesis of aquatic plants, resulting in little difference of chemical properties among monitoring points in the month with more precipitation. Despite that HCO₃^- concentrations at JJQ4 were similar to those at JJQ1 and JJQ2, while the pH and DO at JJQ4 were significantly higher than that at JJQ1 and JJQ2, and the stable isotopes of dissolved inorganic (δ13CDIC) at JJQ4 were heavier than those at JJQ1 and JJQ2. The above phenomenon may attribute to the gas exchange between water and atmosphere because of head fall, which caused that the oxygen in the atmosphere dissolved into the water and the CO₂ in the spring water was released into the atmosphere due to high pCO₂. The hydrochemistry of Jiangjiaquan were controlled by photosynthesis of aquatic plants as well as gas exchange between water and atmosphere. Although the two factors had similar effects on hydrochemistry, they brought different effects on karst carbon cycle. Especially, CO₂ degassing would further weaken the effect of karst carbon sink, at the presence of strong acid such as H₂SO₄. Therefore, it was necessary to distinguish the two factors, in order to accurately estimate the karst carbon flux.