In this paper, LY12 alloy was processed by Plasma Micro-arc Oxidation (MAO) technique. When the aluminum alloy was processed by MAO with positive pulses and positive-negative pulses, the latter turned on a slowly growing speed, low surface roughness and high surface micro-rigidity for the layers. The aluminum alloy was set on both anode and cathode, and was processed by MAO at the same time. The number of positive pulses and negative pulses greatly changed the layers. When four positive pulses and four negative pulses are applied, the layer grows quickly and its surface roughness decreases remarkably. Compared with mono-polar MAO, the aluminum alloy MAO ceramic layers processed by the double-polar MAO grow efficiently, and the layer performances improve at the same time.