Abstract: To explore the influence of elevated atmospheric CO₂ concentration on algae blooms, we studied effects of elevated CO₂ concentration on the growth, physiological and photosynthesis of a bloom-forming cyanobacterium, Cylindrospermopsis raciborskii. The results showed that elevated CO₂ concentration significantly increased the biomass, the maximum photosynthetic rate and photosynthetic efficiency of C. raciborskii. The concentration of CO₂ at 700 mg/L did not impact dark respiration rate and light saturation point of C. raciborskii but had significant effect at 1000 mg/L. The increased CO₂ reduced the affinity of algal cell photosynthesis to inorganic carbon and, the extracellular carbonic anhydrase activity. These results suggest that the increase in atmospheric CO₂ concentration might be beneficial to growth and photosynthesis of C. raciborskii, which may increase the risk of blooms.