Much of the research effort focused on the impacts of elevated CO2 on marine algae but very little work was done on freshwater algae, or on freshwater algal viruses. In this paper, we studied the impacts of elevated CO2 on the infection of a freshwater cyanobacterium (wild Leptolyngbya sp.) by cyanophage PP that have a wide distribution in China. In a 12-month experiment, logarithmic-phase host cells were infected with cyanophage PP at 370 or 740 µatm pCO2 concentrations; the burst size, lysing cycle and proportion of adsorption were measured. The results showed that the proportion of adsorption, and burst sizes of cyanophage PP increased significantly with elevated CO2 concentrations, and the proportion of adsorption increased gradually within the 12 months with the gradual increment of cell width. The result indicated that elevated CO2 concentration may have significantinfluences on the proliferation dynamics of cyanophage–host systems, and some of the influences may increase gradually in a long-term.