Abstract: Karstification connects with global carbon circle closely. The erosion of carbonate can consume CO2 from atmosphere and soil air and convert them to HCO3- in the water, therefore, the feature of karstification carbon sink can be got by way of HCO3- monitoring in the water. The HCO3- concentration from 14 large karst springs and 16 outlets of underground river in Guijiang Catchment are tested on filed monthly and regularly with the alkalimeter made by Merk Corporation. It is found that the average concentration of HCO3- of groundwater is 223.62 mg/L in low water level period (from November to February in the next year), 222.11mg/L in normal water level period (in March and September) and 210.19 mg/L in high water level period ( from April to August). Although the mean concentration of HCO3-in high water level period is lower than that in normal water level period, but the discharge in high water level period is highest, which is twice more than that in normal water level period and 2.8 times as many as that in low water level period, so the carbon sink in high water level period is 4.7 and 2.7 times more than that in normal and low water level periods, and the carbon sink in the three periods accounting for 63.13%, 13.35% and 23.51%, of the whole year respectively.