Water gas shift (WGS) reaction is of great importance for industrial hydrogen production. Using density functional theory calculation, we systematically investigated the catalytic processes of WGS on Fe3O4 (111) surface. Our calculations indicate that the activation barrier of the redox mechanism is much larger than that of the associative mechanism, suggesting the intermediates of WGS prefer to undergo the associative pathway. H atoms recombination towards H2 is the rate determining step with an activation barrier of 1.29 eV, in good agreement with reported experimental values. The results provide rational explanations to the experimental phenomena of WGS reaction, and the improvement of Fe3O4(111) catalyst was also discussed.