Coating carbon on the iron ore powder is a quite promising way to prevent defluidization during its high temperature direct reduction in the fluidized bed reactor, but usually causes excessive carbon content of the DRI production, thus the present study focuses on decreasing the critical carbon content (C-critical value) needed for defluidization prevention by the optimization of the pre-reduction process. It was for the first time found that increasing the metallization ratio of the pre-reduced iron ore significantly could reduce the C-critical value, since it inhibited the carbon consumption and formation of metallic iron during the early stage of high temperature reduction. Based on this result, optimization principle to reduce the C-critical value was further proposed, i.e. increasing the pre-reduction rate (Rpre-reduction) and reducing the carbon deposition rate (R-carbon (deposition)) during the pre-reduction, such as by increasing the temperature and the H-2 mole fraction in CO-H-2 mixture gas. And such principle was verified by the experiment results, where for the iron ore pre-reduced in CO-H-2 mixture gas with the X-H2 >= 0.9 at 600 degrees C, its corresponding C-critical value was less than 3.5 wt.%, much lower than that of other conditions or the reported value in the literature. (C) 2015 Elsevier B.V. All rights reserved.