Quantitative evaluation of the impact of human activities to karst carbon sink not only helps to improve the estimation accuracy of karst carbon sinks, but also is significant for the current research on global climate change. The Wujiang River, the largest river in Guizhou Province, is a tributary of the Yangtze River with a highest proportion of carbonate. Recently, with the fast population proliferation and rapid economic growth, the drainage basin is intensively interfered by anthropogenic activities and shows an increasing SO_4~(2-) and NO_3~- concentration. For investigating the hydrochemical characteristics and the main influencing factors, the hydrochemistry and Carbon isotope in dissolved inorganic carbon (DIC) from 42 and 26 karst groundwater samples were sampled and analyzed during the rainy (July, 2013) and dry (October, 2013) seasons, respectively, from the Upper and Middle reaches of Wujiang River. The contribution of atmospheric/soil CO_2 (CO_(2ATM-SOIL)) to the total HCO_3~- (CO_(2ATM-SOIL)/ total HCO_3~-) were calculated using the stoichiometry method. The results showed that the advantage cations of groundwater and surface water were Ca~(2+) and Mg~(2+), which accounted for more than 55%, and the advantage anions were HCO_3~- and SO_4~(2-), accounting for more than 85%. The concentrations of Ca~(2+) and Mg~(2+) exceeded the equivalent concentrations of HCO_3~-, which the excess of Ca~(2+) and Mg~(2+) cations were compensated by SO_4~(2-) + NO_3~-. The delta~(13)C_(DIC) of karst groundwater in the rainy and dry seasons, ranged from - 14.19 to - 8.27 and from - 12.98 to -9. O9, respectively, with a more depleted value in rainy season. A good positive correlation between delta~(13)C_(DIC) value and [ SO_4~(2-) + NO_3~-]/[ HCO_3~-] indicated that sulphuric and nitric acid might play a relatively important role in carbonate weathering and has an important influence on delta~(13)C_(DIC) value. The average contribution of atmospheric/soil CO_2 (CO_(2ATM-SOIL)) to the total HCO_3~- (CO_(2ATM-SOIL)/total HCO_3~-) is 33.83% for rainy season, and 35.84% for post-rainy reason, which corresponds to a mean decrease of 16.17% and 14.16% compared to natural environmental conditions (50%), respectively. As a consequence, in study area, the equal deficit of soil CO_2 uptake by carbonate weathering reach 32.34% and 28.32% when nitric and sulfuric acids were involved in.