AZ91D magnesium alloys were treated at different time by micro arc oxidation (MAO) in silicate-containing electrolyte. In this work，two types of power loading processes, named P1 mode (constant voltage) and P2 mode (variable voltage ) respectively, were used to prepare the coatings. Varitations of the thickness, surface morphology, phase composition and corrosion resistant of the coatings with treatment time were investigated and the size and amount of the micro pores on coating surface and the surface porosity were studied quantitatively. The results show that the coatings obtained under these two power loading processes had similar evolution patterns in characteristics of microstructure and the extent of which was larger in P2 mode. With prolonging treatment time, the thickness of the coatings was enlarged, with the amount of the total mircro pores and the bigger ones decreasing and increasing, respectively, and the surface porosity of the coatings increased. However, the phase composition of the coatings had no change. The corrosion resistance of the coatings increased continually under P1 mode but it firstly increased and then decreased under P2 mode with treatment time. Comparing these two processes, the P1 mode has an advantage in industrial applications because the MAO coating can grow in a larger rate during the early stage.