The work aims to improve corrosion resistance of micro-arc oxide film of magnesium alloy. Micro-arc oxidization was performed to AZ91D magnesium alloy, and holes in micro-arc oxide film of AZ91D magnesium alloy were sealed by adjusting secondary voltage. Phase, surface morphology and corrosion resistance of the micro-arc oxide film were investigated by means of XRD, SEM and electrochemical test. The film was mainly composed of MgO, MgF2, ZrO2 and Mg2Zr5O12, which were not affected by the secondary voltage. With the increase of secondary voltage, diameter and porosity of discharge micropores on the film first decreased and then increased. Compared with that not subject to secondary voltage, corrosion current decreased by 2~3 orders of magnitude, polarization resistance increased by 1~2 orders of magnitude and corrosion resistance improved significantly when the secondary voltage was applied. At the secondary voltage of 160 V, polarization resistance of the film was the highest and the corrosion resistance was the best. Secondary voltage can seal the pores in micro-arc oxide film of AZ91D magnesium alloy to prevent corrosive liquid from entering into the substrate through micropores and improve corrosion resistance of the film.