In order to research the Na2MoO4 modifying mechanism of micro-arc oxidation (MAO) coating on ZL108 aluminum alloy, the alloy was treated by MAO in an electrolyte with Na2MoO4 addition of 5 kinds of concentration. Surface morphologies, Mo and O elements content on cross-section of MAO coating, valence state of Mo and O element, phase composition and corrosion resistance of MAO coating were analyzed by SEM, EDS, XPS, XRD and polarization curve, respectively. The results show that the oxidation voltage declines with the Na2MoO4 concentration increasing. The MoO42- begins to transform to MoO2 when the temperature of micro-arc discharges area is above 1823.84 K, and MoO2 strains the formation of porous coating on the surface of MAO coating. Therefore, the compactness, thickness and corrosion resistance of MAO coating are improved. In addition, the effect of Na2MoO4 concentration on the phase composition of MAO coating is not obvious.