To reveal the effects of carbonate weathering on chemical weathering processes and its carbon sink effect in river catchment, this paper took the Xijiang River in Guangxi as the research object, which is a typical river that flows through subtropical areas in China. The river water samples were collected from three mainstream sections: Yasngshuo, Zhaoping and Wuzhou, which were analyzed regular monthly and intensive observation during heavy rainfall period from April 2011 to March 2012. The research shows the results as follows: (1) The water of the three mainstream sections is alkali waste water and its hydrochemistry type be - longs to HCO3—Ca. EC (electric conductivity) increases gradually along the mainstream, this phenomenon primarily embodies the control effect to chemical characteristics by carbonate rock that is widespread in river catchment; (2) In a whole hydrological year, the total flux of carbon at three mainstream sections were calculated month by month, and 5.10 ×1012kgCO2/a at the Wuzhou section, 1.55×1012kgCO2/a and 1.80×1012kgCO2/a at the Zhaoping section and Yangshuo section respectively. The intensity of carbon flux is 15 606.13kgCO2/km2 /a, 10 373.61kgCO2/km2 /a and 32 223.08 kgCO2/km2 /a respectively; (3) The flow is the primary factor of karst carbon flux, the phenomenon that rainfall influences flow, further influence the effect of carbon sink, it may be affected by human activity. The change trend of HCO3 - concentrations is different in the water at the three mainstream sections, the possible reason is that the relationship between the HCO3 - concentrations and carbon flux is complex, and it relates various reaction mechanisms. The carbon flux has no obvious correlation with pH value and temperature, it may be caused by the effects that the biolog - ical pump is mainly diurnal variation, pH value and hydrological changes can not truly reflect the influence on the carbon sink by pH value and temperature in monthly scale.