There is no research on sealing treatments for PEO-coated Mg with phosphate sealing solutions containing different cations. In this work, four different cations are added to the phosphate solution for sealing treatment of PEO-coated Mg. The PEO coatings sealed with solutions containing Ca2+ and Mn2+ achieve the best corrosion resistance, since their dense deposition layers can seal the micropores and resist the erosion of the corrosive medium. The physical model and chemical equation used to deposit the formation of the sealing reaction are established by the experiment. They can predict the sealing effects when adding different cations to the phosphate solution. Comparing with other researches which are largely restricted to investigating the effect of phosphate-based sealing treatment on the corrosion resistance improvement of PEO coatings, conclusions here provide better guidance for formulating sealing methods to the PEO coating.