An important aspect of the current global change research is using river chemical composition to reveal the chemical weathering process and its effect of carbon sink. In this study, water samples were collected and analyzed 2~3 times per month from January to December in 2013. The hydrochemistry belonged to HCO_3-Ca type. Ca~(2+) and HCO_3~- were the main cation and anion, which reflected that the hydrochemical characteristics of river were mainly affected by the dissolution of carbonate rock. The concentration of main ions varied with the seasons, which reflected that the crest value occurred in winter, followed by those in autumn and spring, and the lowest value was observed in summer. Due to the interaction of effect of dilution and effect of CO_2, the seasonal variation of Ca~(2+) and HCO_3~- showed that the highest value was in autumn and the lowest value was in summer. The seasonal variation law of other ions should be attributed to the effect of dilution or agricultural activities or combined action of them. Both carbonic acid and sulfuric acid took part in the chemical weathering of carbonate rocks as evidenced by stoichiometric analysis. Besides, the delta~(34)S of sulfate ion of the river waters (delta~(34)S: from 7.65 to 8.55)showed that SO_4~(2-) was originated mainly from oxidation of sulfide minerals in ore deposits and acid rain. Chemical mass balance method was applied to estimate the proportion of HCO_3~- coming from carbonate weathering by sulfuric acid. The result was 28.26%. On this basis, the total carbon flux of carbon (by CO_2 calculation) in Liuzhou section calculated month by month was about 8.95*10~5 t·a~(-1). What's more, the carbon flux showed a positive correlation with flow, which implied that the discharge of catchment was the main influencing factor of carbon flux rather than the HCO_3~- concentration.