HCHO oxidation over Co3O4 catalyst was investigated via a combination of density functional theory (DFT) and microkinetic simulations. DFT calculations indicate O-2 cannot adsorb on the perfect Co3O4 surface and C-H bonds cleavage involves lattice oxygen atoms. On defective surface, O-2 can adsorb on oxygen vacancy to form active oxygen species, which decrease the dissociation energy barrier of C-H bonds. H2O can dissociate into OH group, which can activate lattice oxygen atoms. More importantly, the presence of OH can greatly promote active oxygen diffusion on Co3O4 surface, which is really essential for the second C-H cleavage. Microkinetic simulations show that the two C-H bonds cleavage are the key steps and the overall reaction rates of HCHO oxidation grow at exponential rate with the assistant of active oxygen species. We presents a molecular level understanding to the reaction mechanism of HCHO oxidation, and provides a special focus on the role of oxygen vacancy and water on Co3O4 surface.