Micro-arc Oxidation (MAO) is a new technology for non-ferrous metal surface treatment through growth ceramic layer on the original surface. This article uesed the pulse power supply with a discharge circuit to generated oxide ceramic on the surface of magnesium alloy AZ91D. In order to reveal the influence of the electrical parameters, include current density, voltage, frequency and duty ratio on the film forming of magnesium alloy micro-arc oxidation processing, the orthogonal experimental was carried out under the different electrical parameters for the pulse parameters optimization. Results showed that under the pulse power supply with a discharge circuit, the large arc phen menon can be resolved effectively through reasonable proportion the oxidizing time and the cooling time. The film form increase as the duty ratio increase. The trend of the large arc phen menon significant reduce at the duty ratio is less than 30%. The influence of the frequency on the micro-arc oxide film layer growth can be ignored, but the micro-arc oxidation reaction would be intensified due to the thermal inertia. The results indicate that the big arc phen menon can be effectively avoid, the film grown fast and have a well corrosion resistance at the frequency of 667 Hz or 500 Hz (cycle is 1500 μs or 2000 μs) and the duty ratio of 30%.