The magnetite Fe3O4+delta (0 less than or equal to delta less than or equal to 0.5) of different particle sizes were prepared using wet-heating oxidation method, supercritical fluid dry method and ferrous oxalate and ferric oxalate high-temperature decomposition method, and their structure were characterized by XRD, Mossbauer spectra, IR spectra and STEM. The oxygen-deficient magnetite of Fe3O4+delta (0 < delta < 1) was prepared using H-2 reduction of Fe3O4+delta (0 < delta < 0.5) at 573 K and its properties (lattice constant, magnetism, stability, and reduction ability) were studied ih detail. The activity of decomposing CO2 with oxygen-deficient magnetite of different sizes were compared and the effects of degree of oxygen-deficiency, reaction time, and temperature on the activity of the decomposing CO2 into carbon with oxygen-deficient magnetite were investigated. It is found that oxygen in the CO2 was incorporated in the form of O2- into oxygen-deficient magnetite and CO2 was reduced to carbon, at the same time Fe3O4-delta converted into stoichiometric Fe3O4. The smaller the particle size of magnetite is, the larger the oxygen-deficient degree is, the higher the reaction temperature is, and the higher the activity of decomposing CO2 is.