The formation of the oxidation coatings on the AZ91D magnesium alloy under different processing conditions was studied by optical microscope and the scanning electron microscope. The growth mechanism of coatings was also discussed. The results show that under the test voltages, the process of arcing could be divided to three stages: the first stage is the competing stage between partly corrosion and oxidation, the corrosion begins at the defects of the surfaces firstly, loose oxidation coatings are forming, and at the same time electrolysis of water happens; the second stage is the microcell discharging stage, the surfaces are oxidized integrally and compact oxidation coatings with a few pores are formed, and the electrolysis of water is aggravated; the third stage is the arcing discharging stage, the oxidation process becomes more drastic, the formed coatings are punctured by arcing to form connected pores, and the size and number of pores are increased