Abstract: The effect of elevated atmospheric CO2 on soil available K in paddy soil through FACE（Free air carbon dioxide enrichment）system was studied by comparing the contents of soil available K at different growth stages of the rice-wheat rotation system. The results show that compared to the ambient air treatment, K uptakes of the crops are increased by elevated CO2 due to significant increase of their biomass, while the contents of soil available K of both upper soil layer (0-5 cm) and lower soil layer (5-15 cm) are not decreased. The increase magnitudes of soil available K due to elevated CO2 are 6.3%–22.3% in root rhizosphere and 3.7%–11.2% between crops lines. The increment of soil available K around wheat roots is higher than that of rice. These results indicate that root increased by elevated CO2 is beneficial to soil available K for crops, and K is not a limited factor for response of crops to elevated CO2 in a short time. However, this phenomenon may have negative effects for soils with poor fertility.