Mg alloys, as the lightest structural metallic materials, have many excellent properties. However, the chemical activity of Mg is high, resulting in poor corrosion resistance. Especially, Mg alloys are susceptible to pitting corrosion with great destructiveness and hidden danger. To decline the negative effect of pitting corrosion on the safe service of Mg alloy parts, it is necessary to clarify the pitting corrosion mechanism of Mg alloys. The classical pitting corrosion mechanism of other metals is based on the formation of oxygen concentration cell, whereas cathodic hydrogen evolution reaction occurs on Mg alloys. Thus, the pitting corrosion process of Mg alloys needs to be investigated in detail. In this paper the pitting corrosion process of GW93 cast Mg alloy is monitored in-situ using SVET (scanning vibrating electrode technique) in 3.5% NaCl solution; the corrosion morphologies are observed using SEM (scanning electron microscopy); and the effect of cathodic and anodic potentials on the pitting corrosion process is compared using electrochemical measurements. The results indicate that the microcathodes locate in the outside of corrosion pits and hydrogen evolution reaction occurs there, while the microanodes locate in the interior of corrosion pits and Mg dissolution reaction occurs. The pitting corrosion process dynamically changes with increasing corrosion time. The synergistic effect of the microgalvanic acceleration resulted from second phases and enrichment of chloride ions in corrosion pits is the driving force for the propagation of corrosion pits towards the interior of Mg substrate.