Abstract: The micro-arc oxidation (MAO) coatings were successfully formed on a bulk ultra-fine grained (UFG) AZ91D alloy in a silicate electrolyte, and then the MAO process parameters were optimized via an orthogonal test to improve the surface protection of the UFG Mg substrate. The optimum MAO parameters are 400 V of maximum peak forward voltage, -15 V of peak negative anode voltage, 600 Hz of operating frequency, 30% of duty cycle ratio and 20 min of oxidation time. The MAO coating formed under the above conditions had a lower porosity (8.3%), greater even thickness (19.23 μm), higher pitting potential (-0.199 V), larger Rt value (7 765 kΩ·cm2) and longer proctection time (1.29 times).