Response of soil microbial biomass carbon（SMBC） to different concentration of carbon dioxide was investigated during corn stalk decomposition by incubation experiment in laboratory.The results showed that the addition of corn stalk in soil stimulated the growth of soil microorganism.The SMBC in each treatment reached peak of whole incubation period on first day and then dropped rapidly.SMBC decreased slowly after 90d.Under the condition of different carbon dioxide incubation,the difference among different treatments was not notable during 0d to 15d incubation.SMBC decreased as the concentration of carbon dioxide increased and the differences were significant during 30d to 270d incubation.Soil organic carbon,humus,fulvic acid and humin were positively correlated with SMBC respectively.SMBC was positively correlated with water-soluble organic carbon（r=0.649,p＜0.05） for carbon dioxide concentration of 0.03%,which indicated that normal atmosphere condition is more advantageous to the activity of soil microorganism.