Abstract: Carbonate was added to the silicate system electrolyte to improve the corrosion resistance of the plasma electrolytic oxidation coating on Mg–9Li–3Al (wt%, LA93) alloy. The influences of carbonate on the morphology, structure, and phase composition of the coating were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. The corrosion resistance of the coating was evaluated by electrochemical experiment, hydrogen evolution, and immersion test. The results showed that the addition of carbonate resulted in a denser coating with increased hardness, and the corrosion-resistant Li₂CO₃ phase was formed. Electrochemical experiments showed that compared with the coating without carbonate, the corrosion potential of the carbonate coating positively shifted (24 mV), and the corrosion current density was reduced by approximately an order of magnitude. The coating with carbonate addition possessed a high corrosion resistance and long-term protection capability.