CO_2 cycle process or sources/sinks are not only the basis of understanding and responding to global climate change, but also the core of the current global climate change research. Gas exchange across water-air interface in terrestrial surface water is an important way of nutrient elements (carbon, nitrogen) exchange between aquatic ecosystems and ambient air. Escaping CO_2 gas from surface water is also actively involved in the modern carbon cycle. In the material cycle in karst regions, CO_2 plays a key role in karst processes, driving the formation of karst features. Karst groundwater with high water CO_2 partial pressure (pCO_2) often shows highly positive CO_2 concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO_2 exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO_2 exchange fluxes across water-air interface in the karst surface stream in detail which was fed by Guancun subterranean stream in Liuzhou city, Guangxi province. Closed static chamber method and portable hand-holding CO_2 sensor (GM70) were both employed in CO_2 exchange fluxes monitoring. The results showed that CO_2 degassing was the mainly form of CO_2 exchange across the steam water-air interface. CO_2 degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m~2·h)~(-1) with an average of 445.72 mg·(m~2·h)~(-1). CO_2 degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m~2·h)~(-1) with an average of 159.81 mg·(m~2·h)~(-1). The CO_2 degassing flux in G1site was higher than that in G2 site. CO_2 degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO_2 gas (delta~(13)C-CO_2) found that CO_2 degassing from karst stream might influence air CO_2 carbon isotope near water surface, which resulted in the more negative delta~(13)C-CO_2 value with the increase of CO_2 degassing flux. Significant spatio-temporal variations of delta~(13)C-CO_2 were found, and the delta~(13)C-CO_2 in the rainy season was more negative than that in dry season and delta~(13)C-CO_2 in G1 site was more negative than that in G2 site. As a result of stream CO_2 degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO_3~-, EC and pCO_2 and increase of pH, SIc and delta~(13)C-DIC in the stream.