Plasma electrolyte oxidation films (PEO) on AZ31 magnesium alloy were prepared in alkaline silicate solutions with two kinds of voltage waveforms (constant voltage waveform and escalating voltage waveform). Discharge characteristics and the dielectric barrier layer formation of the films formed under two voltage waveforms were investigated. Structure, composition, morphology and corrosion resistance characteristics of the films were studied with scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and potentiodynamic corrosion testing. The porosity of the films was calculated with Mat lab software. Structure, composition and porosity of the films are correlated with changes in behaviors of spark discharges on the surface in oxidation process. The escalating voltage waveform strongly affects the transformation of Mg2+ from the substrate but not the SiO32- ions in the electrolyte and the compactness of dielectric barrier layer. The films formed under escalation voltage waveform exhibited lower porosity and higher corrosion resistance than the films formed under constant voltage waveform because of less defects on the surface of the films. The voltage waveform change has little influence on the thickness of the films.