In order to get a clear picture for describing the growth process of plasma electrolytic oxidation coatings under escalating voltage waveform, the characteristics of PEO coatings formed at different reaction stages were systemically investigated. The morphology and corrosion resistance of the films were studied by scanning electron microscope and potentiodynamic polarization curves. The uniform, semi-transparent and better corrosion resistance of the oxide films on the magnesium electrode surface were formed owe to the extended anodizing time at the initial stage under escalating voltage mode. After sparking occurred, generated ceramic coatings were brokedown, melted, cooled and solidified continuously, so the ceramic coatings were uniform and dense. It also exhibited superior corrosion resistance.