<p>The study provides the major ion chemistry, chemical weathering rates and temporary and net CO<sub>2</sub> sinks in the Beijiang River, which was characterized as hyperactive region with high chemical weathering rates, carbonate and silicate mixing lithology and abundant sulfuric acid chemical weathering agent with acid deposition and AMD origins. The total chemical weathering rate of 85.46 t km<sup>−2</sup> a<sup>−1</sup> was comparable to other rivers in the hyperactive zones between the latitude 0–30°. Carbonate weathering rates of 61.15 t km<sup>−2</sup> a<sup>−1</sup> contributed to about 70 % of the total. The lithology, runoff and geomorphology had significant influence on the chemical weathering rate. The proportion of carbonate outcrops had significant positive correlation with the chemical weathering rate. Due to the interaction between dilution and compensation effect, significant positive linear relationship was detected between runoff and total, carbonate and silicate weathering rates. The geomorphology factors such as catchment area, average slope and hypsometric integral value (HI) had non-linear correlation on chemical weathering rate and showed significant scale effect, which revealed the complexity in chemical weathering processes. DIC-apportionment showed that CCW (Carbonate weathering by CO<sub>2</sub>) was the dominant origin of DIC (35 %–87 %) and that SCW (Carbonate weathering by H<sub>2</sub>SO<sub>4</sub>) (3 %–15 %) and CSW (Silicate weathering by CO<sub>2</sub>) (7 %–59 %) were non-negligible processes. The temporary CO<sub>2</sub> sink was 823.41 10<sup>3</sup> mol km<sup>−2</sup> a<sup>−1</sup>. Compared with the <q>temporary</q> sink, the net sink of CO<sub>2</sub> for the Beijiang River was approximately 23.18 × 10<sup>3</sup> mol km<sup>−2</sup> a<sup>−1</sup> of CO<sub>2</sub> and was about 2.82 % of the <q>temporary</q> CO<sub>2</sub> sink. Human activities (sulfur acid deposition and AMD) dramatically decreased the CO<sub>2</sub> net sink and even make chemical weathering a CO<sub>2</sub> source to the atmosphere.</p>