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 CO 2 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 CO 2 , i.e., the higher the activity of decomposing CO 2 is. There was specific rapid adsorption of CO 2 on Fe 3 O 4 - δ (δ>0), with oxygen deficiency and this adsorption accompanied with decomposing CO 2 into carbon. Fe 3 O 4 - δ transfers its electron to the carbon in CO 2 ; therefore, CO 2 decomposes into carbon. Simultaneously, Fe 3 O 4 - δ captures the oxygen in CO 2 and converts itself into stoichiometric Fe 3 O 4 ; so, Fe 3 O 4 - δ is deactivated. But Fe 3 O 4 - δ could be regenerated by activating Fe 3 O 4 by H 2 reduction. The reaction of decomposing CO 2 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 CO 2 into carbon on oxygen-deficient magnetite undergoes via two steps: CO 2 ->CO+O 2 - 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.