Emission of CO2 resulting from mineralization of organic carbon in agricultural soil is main pathways of greenhouse gas emission. In this research, effect of biochar on humus, organic carbon accumulation, and metabolic quotient of soil were investigated. Different contents of biochar were added in two kinds of soil with different fertility. According to the results, biochar played a positive role on promoting the formation of soil humus. Contents of humic acids (HAs), fulvic acids (FAs), and humins (HMs) in treatment with biochar increased obviously, which was benefit to the stability of soil organic carbon pool. After 210 days incubation, content of HAs increased 9.8% to 13.6%; content of FAs increased 17.2% to 20.8%; content of HMs increased 20.5% to 22.6% compared with control. Biochar promoted the accumulation of nutrient elements thereby improved the soil quality. In addition, biochar accelerated the formation of carbohydrate, esters, aromatics and other organic macromolecules, which is difficult to decompose by soil microbial, resulting in the stability increase of organic carbon. Moreover, after 210 ‐days incubation, metabolic quotient (qCO2) in soils with 8% biochar decreased 5% to 10% compared with control, suggesting that biochar has positive effect on the ecological function of soil. Therefore, it can be concluded that biochar can affect on soil carbon cycle by changing soil carbon characteristics. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1467–1472, 2015