The properties of reduction and adsorption of magnetite were investigated using temperature programming technique and thermogravimetrical (TG) analysis, and the properties of magnetite were correlated with the activity of decomposing COP into carbon. The smaller the particle size of magnetite, the larger the surface area; and the lower the activated temperature by H-2 reduction, and the faster the activated speed (the speed of lost oxygen), the greater the amount of adsorption of O-2 and CO2, i.e., the higher the activity of decomposing CO2 is. There was specific rapid adsorption of CO2 an Fe3O3-delta (delta > 0), with oxygen deficiency and this adsorption accompanied with decomposing CO2 into carbon. Fe3O4-delta transfers its electron to the carbon in CO2 therefore, CO2 decomposes into carbon. Simultaneously, Fe3O4-delta, captures the oxygen in CO2 and converts itself into stoichiometric Fe3O4; so, Fe3O4-delta is deactivated. But Fe3O4-delta could be regenerated by activating Fe3O4 by H-2 reduction. The reaction of decomposing CO2 into carbon with magnetite belongs to quasi-catalytic reactions, and is equal to one-side reaction of the reversible process of the water gas shift reaction (WGS). Decomposition of CO2 into carbon on oxygen-deficient magnetite undergoes via two steps: CO2 --> CO + O2- and CO --> C + O, the former step is the control step of the reaction speed, and the intermediate product of CO is rapidly converted further into carbon.